Discovering buoyancy. I always have a hard time describing buoyancy. Luckily, Bill Nye does it better.
The boys gathered things from around the house (mostly plastic toys - float!), put them in the water, and recorded data (on papers that ended up QUITE wet).
"No, the pirate ship doesn't fit in the bowl!" (repeat several times).
The foil part was fun - when I asked them to make something that would float and hold something, the redhead barely folded his and put it in the bowl. When I asked him to make something that would sink, he crumpled it in a ball. I could tell he got it!