Thurston Lava Tube - go early and bring a flashlight!
The Thurston Lava Tube is interesting, mostly because it is something different than the rest of the park. But if you go in the middle of the day, you'll get caught behind busloads of tourists. Go early and you'll be rewarded with having the tube practically to yourself. Also, bring a flashlight. There is a second section of the tube at the end you can continue to explore, but you need to light it up yourself.