Je's guide to a fun night in with AIM:Choose a random target by whatever whim takes you. Picking a mind-numbingly mainstream music group is a good way to start if you're stuck.
Message them with: "Hi! Guess who! ;-)"
Let them tell you who you are. Revel in your new identity.
Alternatively, pretend to be an acquaintance of that person. Bag the person a lot.
Carry on the farce as long as possible. Don't just be passive either, take the conversation forward. "Are you still with that blonde?" is a nice line to use.
For maximum amusement value, do the above with a friend. Compare notes.