My parents were visiting a few weeks ago and they bought Jonah a Smithsonian Volcano Kit. We were staying in a campground when we put it together. It makes for a great outdoors activity. I haven't used plaster of paris before (at least not that I can remember) and I really enjoyed it. We all got our hands wet, but Elisabeth particularly enjoyed that part.
Phase 1 complete, we waited a couple of hours before painting it. Papaw got in on the painting too.
I didn't have any measuring cups in the RV so I think our vinegar to baking soda ratio was off a bit. But, overall it was a success.
After the trip, we took the volcano home, and, in the following weeks, it inspired some of our homeschool activities: we read books about volcanos, learned about the Pacific Ring of Fire and watched a documentary on Mount St. Helens. We also did an experiment to see whether baking powder or baking soda had the best reaction with vinegar - as I recall baking soda works best. We erupted the volcano yet again.
We still have it for another eruption in the future when we can review what we learned.