PSA: This week is National Park Week, an initiative sponsored by the National Park Service to celebrate their 100th anniversary this year. Haizhanzheng / Getty Images HBD, National Parks! To honor the occasion, Lonely Planet released a photography book called National Parks of America, out this week, which features all 59 national parks around the country. Lonely Planet In the book, Lonely Planet gives lots of great tips on how to get the most out of your national park experience. Here's their advice on the best time to visit the most popular parks — and the one thing you ~must~ do while you're there. 1. Great Smoky Mountains, North Carolina and Tennessee © Sean Pavone / Getty When to go: The park is open year-round, but it's the most crowded in summer. Fall is a bit chilly, and in winter, roads and facilities may be closed.Must-do activity: waterfall hopping. Abrams Falls (near Cades Cove), Grotto Falls, and Mingo Falls are all great. 2. Grand Canyon, Arizona © Mike Kline / Getty When to go: The South Rim is open all year, but it's really crowded in summer and really cold in winter — which makes spring and fall the best times to visit. (And the North Rim is only open from May to October.)Must-do activity: rafting. Trips range from about four to ten days, complete with riverside camping. 3. Rocky Mountain, Colorado © Carl Finocchiaro / 500px When to go: It’s open all year, but roads are often closed due to snow from mid-fall through winter. The park is most crowded in July and August, when the beautiful wildflowers are in bloom. Must-do activity: mountain climbing, especially to the top of Longs Peak, the only '14er' (a mountain higher than 14,000 feet) in the park. If you're not a serious climber, there are lots of other beautiful trails, too. 4. Yosemite, California © Nae Chantaravisoot / 500px When to go: The valley in the park is open all year. But areas that have high elevation — like Glacier Point Road — all close after the first major snowfall in fall. They don’t re-open until the following year, in late spring or early summer.Must-do activity: rock climbing. Serious climbers hang out at Camp 4 (near El Capitan in Yosemite Valley) from spring through fall, and at Tuolumne Meadows, which is filled with bouldering opportunities, in summer. 5. Yellowstone, Idaho, Montana, and Wyoming © kwiktor / Getty When to go: The best times to visit are early spring to early summer, and late summer to early fall, as Yellowstone’s roads are really crowded in July and August. Must-do activity: hiking. Uncle Tom’s Trail is a must. 6. Zion National Park, Utah © Benjamin Schaefer / 500px When to go: High season is May through September, but temperatures often get up to 100°F during that time. May is good, though it can get buggy; fall is cooler and ideal for hiking.Must-do activity: canyoneering. Zion is known for its "slot canyons," which are narrow canyon that are deeper than they are wide. Two of the most well-known slot canyons are the Narrows and the Subway. 7. Olympic National Park, Washington © Westend61 / Getty When to go: Most entrances are open all year, but roads may be closed due to weather — and many areas of the park are actually unreachable in winter. It also rains the most in December and January; at high elevations, it can even snow all year. Summer is relatively warm and dry, but it can get rainy, too. Must-do activity: soaking in hot springs. Sol Duc, a resort in the park, has three mineral hot springs pools; day rates are $13.50. From there, you can hike to Sol Duc Falls, which is an easy three-mile hike from the resort. 8. Grand Teton National Park, Wyoming © Ferran Traite Soler / Getty When to go: The park is open year-round, but, like most high-altitude national parks, it's way more crowded in the summer, when the wildflowers bloom and temperatures are pretty comfortable (around mid-70s F). Winter is just beautiful, though some roads may be closed due to snow.Must-do activity: cycling. There is a new car-free path that runs from Jackson to Jenny Lake. Rent a bike at Moose, and bike through the valley — and don’t be surprised if you see a moose munching on grass along the way (especially if you're riding in the morning). 9. Acadia, Maine © AppalachianViews / Getty When to go: The park gets the usual July/August rush, but it’s just as pretty (if a bit colder) in May, June, and September. October's beautiful, too — the leaves are on ~fire.~ Must-do activity: high tea at Jordan Pond House in the summer. The house is famous for its wooden chairs and warm popovers with jam. Having tea there is one of the park's longest-standing traditions, dating back to the 1890s. 10. Glacier, Montana © Kan Khampanya / 500px When to go: It's best to go in late August or early September for the best combo of pleasant weather and fewer people. Not surprisingly, Glacier is pretty cold. It's actually freezing for most of the year; many roads are often shut down after the snow comes in early fall, and many facilities don’t even open until July. Must-do activity: glacier viewing. Glaciers are actually endangered right now; the park has about 25 glaciers in comparison to the 150 it had back in 1850. 11. Cuyahoga Valley, Ohio © Howard Grill / Getty When to go: The park is open year-round, but spring (when wildflowers erupt) and fall (when foliage is at its peak) are the most beautiful times. Must-do activity: scenic touring. Hop on the Cuyahoga Valley Scenic Railroad, which has been traveling through the park since 1880. 12. Joshua Tree, California © sborisov / Getty When to go: Most people go in spring, when the wildflowers are in bloom. But temperatures are mild in both spring and fall, too. Winter can be cold, but on the upside, it has clear, starry skies.Must-do activity: rock climbing. Outdoor outfitters in the town rent and sell climbing gear, and they also teach essons. The park’s climbing ranger also holds open morning coffee sessions during the climbing season (roughly October to April) to answer any questions you may have. 13. Hawai'i Volcanoes, Hawai'i © Mike Rose / 500px When to go: It's open year-round, but winter isn't ideal; the park often experiences wetter, cooler temperatures during that time. Before visiting, check for eruption updates, which can close some viewpoints, roads and trails.Must-do activity: lava viewing. Ask at the visitor center if it’s even possible to see Kīlauea’s lava flow inside the park when you go, because it's really unpredictable. 14. Bryce Canyon, Utah © Chris Martin / Getty When to go: Like most of these parks, summer is crowded in Bryce, so spring and fall are good choices (though they can be pretty cold at high elevations). In winter, the park is beautifully snowy, but many services are closed as a result. Must-do activity: stargazing. The park is super remote, which makes its night skies insanely clear for stargazing. You can see an average of 7,500 stars here, compared to only 2,500 in most parts of America. 15. Hot Springs, Arkansas © zrfphoto / Getty When to go: The park is open all year, but, like the others, it gets busy during the summer, especially during July. Winters are mild, and wildflowers begin blooming in February.Must-do activity: bathing. People have been enjoying the hot springs here since 1912. The full traditional treatment takes about one and a half hours. National Parks of America (Lonely Planet) is out now.