The funniest thing is when a cute girl comes in to the gym with her chubs boyfriend and when he leaves she stares at you the whole time, then when she sees him coming back she looks away. If you start to lick your lips you can really freak them out, especially if they think you don't notice.
Another fun thing to do is if a guy brings his gf to the gym and tries to show off by doing dumbell press with 60's you go right beside him and pick up twice as much weight and watch his girlfriend stare at you. 1 time a guy I didn't like brought his gf to the gym and I followed him around everywhere he went for an hour. He got so pissed off, but he couldn't do anything.