Live food is a classic one. Very stimulating. Objects outside the tank is good too-- i've even heard of someone rotating plastic animal toys (the ones from the dollar store) around the tank. You can also rearrange their decor with each cleaning or even float a ping-pong ball on the top of their tank for them to push around. My betta takes great interest on anything I drop beside his tank on my desk (my, coat, bath towels, my purse, etc) and also likes fighting with my pipette (i squirt it at him under water lol).