Some people use water bottle caps. They put treats in them and float them in the water, then the fish gets to figure out how to get the treats out. Others find Moss balls to be great entertainment for their fish, they like to lay on them and roll them around. Almost anything that is safe to be in the water with fish can be used as a toy. Others have found that taking plastic mesh or pvc pipes and making tubing mazes is a lot of fun for their fish.