It's not legal to place a cache in a National Park, but there's nothing wrong with making the entire park into a cache.
But entire parks are too big to qualify, so how about just the headquarters building?
The usual rules apply:
- You must submit a photograph of yourself and your GPS in front of the sign identifying the location as the headquarter of the park
- Only one entry per park and only one entry per geocacher
- Post the coordinates of the sign
For more information on the US National Parks, try the park service web site at National Park Guide