National parks, it's been said, are the best idea we've ever had. But you know what's even better? Free admission at national parks.

National Park Service sites in Arizona and elsewhere will waive entrance fees this Thursday through Sunday (August 25-28) in honor of the Park Service's 100th birthday, which is Thursday. And Arizona's most well-known national park, Grand Canyon, has some special events planned for visitors.

From 9 a.m. to 5 p.m. Thursday through Saturday, you can stop by Centennial Central at the South Rim train depot. The free event will feature interactive booths from local organizations and park partners, park officials said in a news release. There also will be free birthday cake at 1:30 p.m. Thursday.

Also at the train depot is an Amtrak "Trails and Rails" exhibit, open Thursday through Sunday from 9 a.m. to 6 p.m. It'll include model trains and activities for kids, and it's also free.

Additionally, the park will host a naturalization ceremony for 15 new U.S. citizens at 10 a.m. Thursday at Mather Point Amphitheater. The ceremony is one of 100 being held at Park Service sites in 2016.

For additional information about fee-free days at Grand Canyon and other national parks in Arizona, click here. Note that fees for camping, reservations, tours and concessions still apply.