I have things such as a thermometer suction cupped to the inside of the tank. My two RES love snapping it against the side of the tank to get attention. If they get bored or don't get attention, they pull it off the float it around.
So, I went to Walmart last night and got them a ping pong ball (they are not near large enough to even begin to get their mouth around it). They went nuts over it as soon as I put it in. I think they thought it was food at first. Once they realized it wasn't, one turtle gets on each side of it, and they bite it back and forth. Or one backs the ball into the corner and bites it to himself. And they had just begun bumping it with their feet this morning. It is so funny to watch! And no smacking thermometer sounds!!

