Kissing or making out gets me the most. When I kiss my partner passionately, we're normally pretty hands on so digging her nails in or biting my lip is amazing. Sometimes we just tease eachother with words until we can't take it.
Once I played a really fun game with my boyfriend at the time, we kept rubbing against eachother and forcing the other one to be horny but never did anything directly. That was insane.
You could also try surprising them, especially by coming up behind them and whispering in their ear