Naturally, I'm going to start with my favorite volcanic feature. Willow Mountain,
We didn't get a chance to see it up close (which I really wanted to do, but I was afraid someone was going to come out with shotguns and run us off if we trespassed on their land, not to mention we were running late to get back and cook dinner). The best part about Willow Mountain is the spectacular columnar jointing, which you can see better here:
This is actually not a very well-known example of columnar jointing in the US, from what I can tell; other places (like Devils Postpile in California, Devils Tower in Wyoming, and Sheepeaters Cliffs in Yellowstone) seem to get more recognition. I highly recommend visiting this one, though, if only because it's in an area that's absolutely teeming with volcanic features.