From April 22 to April 26, every national park in the country is offering FREE admission. It's National Park Week. National parks are a good deal any time of the year – especially as 268 of the 401 national parks never charge an entrance fee.
It costs $25 to get into Yellowstone. But that's good for a car full of people and for up to a week of exploration. Still, if you're planning a family trip, hitting popular parks like Grand Canyon, Bryce Canyon, and Zion, you'll save $75 if you go next week.
All year long, you can buy an $80 National Park Pass that gives you a year of unlimited admission.
There are a few other free days this year:
August 25 - the birthday of the National Park Service
September 28th, National Public Lands Day
Veteran's Day Weekend in November
Crowds will obviously be a little bigger during National Park Week, but there are also special programs put on by on the rangers. Check nps.gov for the details on your favorite park.