You could put a few of those feeder minnows or a feeder goldfish in there to keep him busy. How big is the tank? Do you have any powerheads providing any currents?
We had a turtle setup at work where one side of the tank had a nice current to it, in the middle was the basking area, and on the other side was a more "still" area. Basically the turtles could choose where they wanted to go. One downside to aquatic turtles is that they need a lot of space to really be happy...