Annika brought in a wetsuit for her news when she was Star of the Day. We felt the spongy material - it had air pockets in it to keep it afloat.
Celestia brought in her dad's sinkers from fishing. There were all sorts of sizes. Celestia's dad caught a stingray the other day.
Have a look at the Sink or Float websites on the 'Website' page. The Cookie Monster floating experiment is hilarious!