Simple science is the best. It's cheap and easy, with just a few supplies, and is wildly impressive. These swimming raisins are the perfect example.
What You Need
Clear soda (we used Sprite)
Tall, clear glass
What To Do
Fill the glass 2/3rds full with the soda.
Drop several raisins into the glass.
Watch bubbles form on the raisins at the bottom of the glass. As this happens:
1. the raisins will rise to the top of the glass
2.. the bubbles on the raisins will pop when they break the surface
3. the raisins will sink back to the bottom
This will happen again and again and again - until there's no more carbon dioxide gas (aka "fizz") in the soda.
The raisins drop to the bottom of the glass because they are denser than the soda. However, when carbon dioxide bubbles form on the surface of the raisins, the bubbles (which are less dense than the soda) cause the raisins to float to the surface. When they break the surface, the bubbles pop, and the raisins descend again.