2009, I bought the Annual Pass and went all over the US visiting parks as a reason to ride. I ended up in 42 states and 168 parks. It was an absolute blast and I even learned a bit more US history, if you're serious about doing it, there's even an IronButt Association certificate available if you visit 50 parks in 25 states within 12 months. It's called the National Parks Tour.
All the parks are listed at http://www.nps.gov
. For the IBA ride rules.