Lassen Volcanic National Park
Lassen Volcanic National Park is one of the least known/visited national parks in the continental United States, and I'm really not sure why. It has lots of volcanic activity, beautiful scenary, and seemingly lots of hiking area. It was really just a stopover for me on my way up to Mount Rainier, but I will definately need to come back and do a multi-day backpacking trip here at some point!
