America the beautiful.

PSA: This week is National Park Week, an initiative sponsored by the National Park Service to celebrate their 100th anniversary this year.

HBD, National Parks!

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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.

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.

Lonely Planet

1. Great Smoky Mountains, North Carolina and Tennessee

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.

© Sean Pavone / Getty

2. Grand Canyon, Arizona

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.

© Mike Kline / Getty

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