There are a million reasons you should visit a National Park as often as possible. The reasons are all pretty obvious, but that doesn't make them any less valid. The Parks are simply some of the most gorgeous places you can spend time inside the US.
Fortunately, five times this year, the National Park Service will waive admission fees and let visitors in for free. The first of those days will arrive on Monday, January 20 in honor of Martin Luther King Jr. Day. You'll be able to get free entry to more than 400 National Parks at the end of the long weekend.
The annual free days often inspire some people to ask why National Parks aren't always free. (Something, something, tax dollars, something, something.) Well, the parks are underfunded, and it's not easy to care for, preserve, restore, and staff these beautiful spaces.
Nonetheless, many parks are free or host free activities. For instance, you will have to pay to get a guided tour through the absolutely gorgeous cave systems at Mammoth Cave, but the surface activities -- biking, hiking, stargazing, generally enjoying the beautiful surroundings -- are free. (Also, the fees at many of the parks help maintain the parks that do not charge.)
Moreover, the entrance fees are pretty damn reasonable. For instance, you can get an entire carload of people into Zion for $35. As many people as you can fit in the car -- clown cars are still cars -- will be covered by that fee, and it gets you into the park for seven days. That's basically the equivalent of buying a cheese sandwich or sno-cone at a Disney theme park. Plus, the adrenaline rush of a roller coaster doesn't hold a candle to walking across Angels Landing.