I'm taking my recent trip out of order. I wanted to start with one of the things we did AFTER Yellowstone. A place in Idaho called Craters of the Moon. When you look at it from a distance, it looks like it's got green stuff growing. When you get there, you see just how much of it is "barren lava field". You can explore several of the cones that are left behind, but stay on the trail! You can see where lots of feet walking across have damaged the area too.
And for those adventurous enough to try, there are caves you can explore and a lava tube you can walk through. I recommend a flashlight, though! The caves can get very dark.