When you go up to someone tanning on the beach, and without them noticing you (hopefully they're sleeping or something), you write an inappropriate word or phrase on their stomach or back with 70 SPF (or higher if possible) sunscreen.
"The chick got so pissed when I misprinted her yesterday!"
When you put your hands together and roll them down a friend's back unexpectedly in a quick unexpected motion, hopefully making them jump or yelp.
"I've had enough, dude! Stop Pinwheeling me!"