We've been to a number of the fossil sites, there are good pay sites near Rawlins WY too. I've noticed several people mention Lava Beds, usually when Lassen is mentioned. The big draw back is that you have to enter California to get there.
We've visited a couple of state park Indian Mound sites in the east.
We've been to a number of Military parks and even with knowledge of the battle, I'm a military history buff, I find the parks underwhelming, They need to come up with a better way to present the history. A stone monument saying the 6th Volunteer Wisconsin lined up here doesn't work. I suspect the topography has changed a lot over 150 years too.
Wiki has good lists of all the parks.
