One of my other friends printed out a picture of a penis that I put under my roommates pillow inside the pillow case. I thought he would discover it when he went to sleep when he heard the paper make noise but he didn't. But he found it a week later, and I even left him clues suggesting his sleeping head was very near a penis
I also sometimes watch porn while he is in the room and it gets him really pissed off, especially when I use the volume