Grandpa fashioned a volcano using a pizza box, some cardboard, tape, and a film canister. Film canister? People still have those? Thankfully, Grandpa is not one to throw potentially useful things away. Grandma provided a large supply of baking soda, vinegar, and even made a special trip for red food coloring for the "laba," as Eli called it. I tried to convince his it was lava, but he was not going for it.
You can also create more of a lava explosion. To do this, put baking soda in the film canister, add the vinegar, cap it QUICKLY, and shake it, and place it in the volcano. If you do this, make sure to have everyone stand at a safe distance.