(CNN) — The US National Park Service is celebrating a birthday -- 103 years to be exact. And you're invited to the party.

Here's the great thing about this shindig: It's free!

On Sunday, August 25, you can enter any of the 61 national parks and not fork over a single penny in entry fees. The list of 61 includes the big-name destinations : Yellowstone. Grand Canyon. Everglades.

Just a bit about fees: While about two-fifths of the national parks are free to enter all year long (Great Smoky Mountains, for example), the rest do charge entry fees (Acadia and Yosemite, for two) except on these special days.

In all, the NPS oversees 419 places you can visit, of which 112 charge a fee. (All are free on Sunday.)

You can click on this link for an alphabetical listing of which national parks and other NPS sites have a fee.

History behind the national parks

You can thank President Woodrow Wilson for signing the National Park Service into being.

On August 25, 1916, President Woodrow Wilson signed congressional legislation creating the National Park Service.

World War I was raging in Europe, one year before the A little historical context: In 1916,World War I was raging in Europe, one year before the United States entered the war. That shows the value the federal government placed on preserving wilderness areas back then even during tumultuous times.

So a free day at the park is not only a way to have fun and exercise on Sunday; it's also a way to honor an important part of US history.

Some ideas of what to do:

Go on an adventure

Zion National Park in Utah makes for an excellent hike. Marc Neidig/NPS Photo

This is fine opportunity to go on an outdoors adventure. Just this just a small sampling of your options:

If you like to hike, The Narrows at Zion National Park in Utah is an excellent choice.

Are you more of bicyclist? Acadia in Maine is superb idea.

Or grab a paddle and kayak in California's Channel Islands.

Related content National Park adventures: Best parks to get your thrills

See our newest park

You don't have to be on an ocean to get incredible dunes and beaches. Just be in Indiana.

Folks in northern Indiana and the greater Chicago area might want to mark the 103th birthday with a trip to the baby in the bunch: Indiana Dunes National Park.

It joined the elite 61 really recently -- 2019 in fact.

What to do there? Take a dip into Lake Michigan, scout for rare birds, fly a kite, ride a bike or just take in the sandy magnificence of tall dunes.

Related content The glories of Indiana Dunes

Most popular parks

Glacier National Park was No. 10 on 2018's most popular list. Shutterstock

So it's human nature to want to hang out with the cool kids. CNN Travel has a listing of the most popular parks from 2018. No. 10 on the list is Glacier National Park in Montana.

And there are nine more parks that had even bigger attendance. Check out the list if you want to find out where crowds are.

Related content America's 10 most popular national parks

Stay safe when you visit

A grizzly bear mother and her cub walk prowl Yellowstone. Do not approach bears in the wild. Enjoy from a respectable distance. KAREN BLEIER/AFP/Getty Images

While getting outdoors is a good thing, let's remember: It is a wilderness.

Know something about where you're going before you head out and read up on how to stay safe when you get there. A lot of it is just plain common sense:

-- Don't approach animals. Especially bears.

-- Remain on marked trails.

-- It's hot this time of year. Bring water and stay hydrated.

Related content Stay safe out there -- 7 tips you need to know

Help finding a park

Sometimes, you just have to do what's convenient. And for this birthday party, that means heading to the national park that's closest to you.

The park service has a handy interactive that locates parks by state

Two more free days in 2019

So if you have urban obligations this Sunday, you'll have more chances to visit any national park for free this year.