Recently, I saw a tip on Pinterest that said you could grow your own celery at home, not with seeds, but by planting the root end in some soil. So just for fun, I threw some dirt in a pot, and stuck the end in about an inch or so down. Note I did not soak the end in water overnight as is directed in a lot of posts. Within 3 days I had little green leaves. Above is what my plant looks like after 17 days. I think that is pretty impressive growth, and even if you have no interest in growing your own celery, it would make a great science project for little ones based up how quickly it grows.
Here is a great website that lists a total of 5 foods that you can re-grow, I bet there are others too. Green onions are next on my list to try. 5 Foods