Coming up with simple projects can be hard. There are tons of bloggers out there with great ideas, but they often use things I don't have on hand. One day, when I couldn't figure out anything else to do, we decided to pour some baking soda on a baking sheet, dye some vinegar and then use a medicine dispenser to shoot the vinegar onto the baking soda and watch the reaction.
Joshua loved it. He did it until all the baking soda was wet with vinegar and there were no more eruptions to be had.
Sometimes it's the simplest things that are the most fun.