My boy Fizz likes to sleep wedged in between the thermometer and the suction cup, either on to or under it. He has a swim bladder issue that is not clearing up, so wedging himself in there like that allows him to sleep without floating to the top. His second place he spends a lot of time is inside his pineapple.
My girl likes to sleep behind her heater, midway down, I guess that's the warmest spot, but she doesn't stay there very long, she's almost always on the move. She hardly ever uses her pineapple and is generally out in the open except for the short periods of time behind her heater. Sometimes she sleeps on top of her plant bulbs, but I don't see this very often.