Don't neglect the balls! NSFW

If your partner has a penis, don't underestimate the power of the scrotum and testicles as devices for their pleasure. Try putting a finger in the space between the testicles. Be careful not to put too much pressure on the balls themselves though, or it can hurt. If you're comfortable with it, try very gently putting one ball in your mouth if you're in the process of playing with the penis. You might be pleasantly surprised by the reaction. Just be sure to stop if they tell you it hurts, of course.