Get Out! It's Almost Earth Day.

April 21 and 22 are two of the Sierra Club's favorite days. First comes Club-founder John Muir’s birthday on Sunday, followed by Earth Day on Monday, one of the few major holidays not synonymous with acquiring more stuff. In that vein, the Club’s "Get Out!" campaign is all about acquiring experiences.

The Sierra Club’s Earth Day site has a calendar of 500-plus things to do across the country, with events leading up to and beyond the official holiday. Events run the gamut from film screenings and jazz shows, to backpacking and habitat restoration.

Here are 17 of the many ways to celebrate this coming Sunday and Monday:

6 of America's Most Dangerous Hiking Trails

With temperatures rising, many of you are gearing up for a vacation with family or friends. You might be dreaming of the world's most stunning canyons or America's newest monuments, but we know that some of you are also looking for a serious challenge this year. For those who are brave enough, prep those hiking boots, gather your supplies, and tackle some of America's most challenging and scenic hiking excursions.

Mist Trail, Half Dome, California

Tucked away in world-renowned Yosemite National Park, the Mist Trail attracts thousands of visitors every year to climb to Half Dome's 8,836-foot-high peak. Hike through beautiful pine forests, bustling waterfalls, and what seems to be a vertical staircase before reaching the steel cables that will take you the last 400 vertical feet to the top of the dome.

Even with cables to assist, the final ascent to Half Dome requires extreme strength and is only for the brave at heart. Without the cables, the hike to the summit of this beautiful landmark would be virtually impossible. And even with this assistance, there have still been unfortunate causalities on this Californian adventure. Since 1995, six deaths have occurred at Half Dome -- one when a hiker was attempting to pass other climbers on the cables.

7 of the World's Strangest Flowers

For most of the U.S., winter is finally loosening its icy grip. Besides freshening your wardrobe, cleaning house, or planning your next national parks trip, why not celebrate by admiring some flowers and plants, perhaps the most telltale signs of spring's arrival? Sure, you can stop and smell the roses, but why not also marvel at the rafflesia arnoldii, touch-me-not, Eastern Skunk Cabbage, corpse flower, voodoo lily, and Hydnora africana? While you could trek across the globe to view these bizarre blooms, in some cases, you need only venture as far as your local botanical garden.

Rafflesia arnoldii: Weighing up to 15 pounds and measuring more than three feet across, rafflesia arnoldii is the largest individual flower on Earth. While it boasts fleshy, burnt-orange petals dotted with whitish pustules, it lacks leaves and roots, parasitizing the woody Tetrastigma vine in the rainforests of Borneo and Sumatra.  Impressive still, Rafflesia arnoldii emits a fetid odor that some compare to decaying flesh, earning it the nickname "corpse flower."

For such a bold blossom, the Rafflesia arnoldii remains elusive, spending most of its time hidden within its host's stems and roots. The flower appears when its buds break through the Tetrastigma vine's bark. A cabbage-like head develops, eventually blooming and staying open for only about five days. Rafflesia arnoldii's rank smell lures carrion flies. The flies crawl down the male flower's central chamber toward the anther, which transfers pollen onto their backs as they brush against it.  They then carry the pollen to a female flower. The projections emerging from the flower's center may help radiate heat and waft the carrion smell.

4 of the World's Most Magical Treehouse Hotels

From staring mesmerized at the sun-dappled, leafy boughs arched overheard, to peering giddily at the world far below—few childhood experiences are as magical as playing in a treehouse. Even if you’ve never set foot in one of these idyllic hiding spots, consider ditching the stuffy hotel room and venturing on a limb for your next trip. Not only does living amid the branches make for a whimsical getaway,research shows that it could boost your health, too.

Today, we journey around the world to visit some of the world's most magical treehouse hotels. These arboreal abodes are upgrades from the nailed-together scrap wood most of us are familiar with. But whether you seek quiet solitude or a chance to let your inner child run amok, you’ll likely find it, even in the most modern of these lofty dwellings.

Tree House Lodge, Limón, Costa Rica

Hidden in the Costa Rican rainforest, the Tree House Lodge (pictured above) merges luxury and sustainability into a tree hugger’s paradise. Sitting beside the pristine Punta Uva beach, the lodge features three bungalow-style dwellings, as well as the two-level Tree House, built around a twisting, hundred-year-old sangrillo tree. Visitors will find a bedroom and kitchen on the Tree House’s first floor, while a walk across a wooden, palm-thatched suspension bridge will lead them to the upper-level master bedroom, an ideal spot for watching howler monkeys. Bungalows accommodate up to five people, while the Tree House sleeps up to six.

5 Fascinating Animal Mating Behaviors

We'll assume you've gotten the birds and the bees talk, so without ado: Animals reproduce, usually (though not always) through sex. And while that might evoke scenes of cuddly coitus, à la March of the Penguins, procreation isn't always so pretty. It can be strange, scary -- even downright deadly. But behaviors that might seem bizarre in the boudoir are actually completely normal in nature. Today we bring you five fascinating animal mating habits and explain why natural selection might sometimes favor the kinky. 

Banana slugs:  They might look cute, but these bright yellow critters, which inhabit damp, coniferous forests along the north Pacific coast of the U.S., take tough love to a new level. They’re also enormously endowed. As an adult, the banana slug measures roughly 6 to 8 inches in length -- and so can its penis when erect. The organ emerges from a genital pore on its head.  Since slugs, like their snail cousins, are hermaphrodites, the banana slug also has female organs.

Banana slugs begin their tryst with some rough foreplay, lunging, biting, and hitting one another with their tails. Then they curl around each other, like two chubby, slimy commas, and insert their penises. Sometimes one partner gives sperm while the other receives, but usually they exchange sperm. The partners can remain enjoined for several hours. Normally, they then retract their organs and crawl on their merry way. But things can take a grisly turn.

Meet America's 5 Newest National Monuments

President Obama signed five more places into the sacrosanct fold of national land-hood on Monday under the nearly 107-year-old Antiquities Act.

Make room for the Harriet Tubman Underground Railroad National Monument, the Río Grande del Norte National Monument, the San Juan Islands National Monument, the Charles Young Buffalo Soldiers National Monument, and the First State National Monument on your bucket list.

First we'll tell you a little bit about the path to protection for national monuments, then we'll show you why you should visit these five new destinations. 

The Waiting Game

The Antiquities Act lets the president make national monuments of public lands with "historic or scientific" interest, and fast.

While Congress can establish national monuments, too, fast is unlikely — which is just what Antiquities is for, according to Meghan Kissell, campaign communications director for the nonprofit Conservation Lands Foundation.

See: the Grand Canyon.

"There were tremendous fights for years about what to do with that area," Kissell said. "By the time it was designated as a national monument, there had been bills in congress for a dozen year or so. There were a lot of people arguing about what the value of it was."

President Theodore Roosevelt secured the Grand Canyon as a national monument in 1908 (two days after Muir Woods). It became a national park in 1919 via Congress, which alone has authority to establish national parks.

Astronomy in April: A Pair of Comets

Two comets are in the nighttime skies in April, one lingering after sunset and one appearing before sunrise. Comet PanSTARRS made its debut appearance in the Northern Hemisphere in March as it began to appear in the sunset’s glow midmonth. The comet is dimming as it moves out of the solar system, so use binoculars or a telescope to try to track it down. In April it will be crossing through the region of Andromeda, heading toward the North Star.

Comet PanSTARRS's biggest event of the month will be on April 3, when it passes a few degrees from the Andromeda Galaxy, M31. Binoculars or a wide-field telescope will give the best view of this fuzzy pair of objects.

Comet Lemmon is the second good comet of the year, also beginning its show in the Southern Hemisphere before moving into northern skies around April 19. The comet is expected to shine around 4-5th magnitude, similar to a dim star.

Both Comet PanSTARRS and Comet Lemmon will occupy the same region of sky but at different times. Comet PanSTARRS passes through Pisces, moving from left to right in March in the west at sunset. Comet Lemmon also passes through Pisces, moving from right to left in late April in the east at sunrise. Both comets are once-in-a-lifetime for Earthly observers, with Comet Lemmon’s first chance of returning 11,000 years from now, and Comet PanSTARRS 110,000 years from now, if it returns at all.

9 of the World's Most Breathtaking Canyons

Although there is only one that has been named as such, our planet is home to many grand canyons. From steep cliffs to narrow valleys, each evoke a sense of wonder and amazement while documenting thousands of years of geological history. Although each is unique in its own right, we have found nine canyons that are must-sees for any nature lover. But beware, if you have a fear of heights, you may feel uncomfortable just gazing at these pictures, as some of these grandiose layers of rock plummet to depths of close to 10,000 feet. 

 

Green Stars: 5 Luxury Eco-Hotels and B&Bs

With the summer months fast approaching, many of us are starting to plan vacations with family or friends. But wait! Before you book that five-star hotel room, did you know that hotels contribute more than 60 million tons of CO2 emissions annually? U.S. hotels spend over $7.5 billion on energy and generate 1.9 billions pounds of waste each year. Thankfully, there are some amazing alternative lodging options for the more environmentally concerned. And they don't require you to sacrifice that five-star, luxury ambiance. Best of all, they are located in vibrant locations, surrounded by tons of eco-activities.

Sordid San Francisco Falcon Drama

Watching the Santa Cruz Predatory Bird Research Group's Falcon Nest Cam is a popular worktime timewaster educational activity in the Bay Area. Up until last week, it was a peaceful domestic scene: Peregrine falcons Dan and Diamond Lil had produced four eggs in the nest on the 33rd floor of the headquarters of Pacific Gas & Electric. Then all hell broke loose. Here's how the SCPBRG reports this avian Downton Abbey:

March 14, 2013: About one week ago, Diamond Lil disappeared. We do not know her current status, just that she has not returned to the nest ledge. A new female, dubbed "Cher," has been visiting the nest ledge but does not seem inclined to share in incubation duties. Dan has been doing all the incubation but he must also hunt for food to eat. The eggs have been uncovered during these absences and incubation has been uneven. The eggs need about 12 days more incubation. Will the eggs hatch? Will Dan continue to make a heroic effort at incubation? Will the new pair recycle and lay a new set of eggs? We will soon know. 

Watch for the weekly update on Monday here. If you don't want to wait for Netflix, you can join the San Francisco Peregrines Discussion Forum.

PAUL RAUBER is a senior editor at Sierra. He is the author, with Carl Pope, of the happily outdated Strategic Ignorance: Why the Bush Administration Is Recklessly Destroying a Century of Environmental Progress. Otherwise he is a cyclist, cook, and father of two. Follow him on Twitter @paulrauber