A fun activity we did a while back was to make some boats and then take them in the bath to make them float. We did two different kinds, which was a good way to compare which floated better, which would hold something better, which was faster, etc.
The first set of boats were made out of halved styrofoam egg cartons. We decorated them and added popsicle stick flags.
We also tried using the lid of a plastic take-out container, which Love Bug decorated with markers.