Another pinner says: How Many Drops of Water Can a Penny Hold? My daughter did this as her science fair project last year and it was very popular. She even used water variations (Kool-aid, salt water, etc).
Playful Learning: 9 Apps for Young Scientists. These are great because you can integrate technology into your science lesson. The students can enjoy a game every so often and not know they are learning at the same time :-)