The boys had a blast doing their own little volcanoes in a cup. You can find this and more experiments with different mixtures here.
We used small cups, white vinegar, baking soda, food coloring, and a tablespoon (any spoon will work), and a cookie sheet to keep the mess contained. All things that were hanging around the house already.
I filled the cups about half way full of vinegar and then we added a little food coloring to make it more fun. Then I let the boys take turns putting in some baking soda. They had fun watching the "explosions".
I had five cups that we had fun with. Then we dumped them out and did it all over again because that wasn't enough fun. In fact, my 2 year old cried when I told him we were done for the day. Needless to say this activity was a success!