Recently I have found it funny to say, "High Five" to people after hearing them tell me about something they think is a cool accomplishment. The fun part for me is to drop my hand back down right as they go for the high five slap. It leaves them hanging there with their arm up with a look of confusion and disappointment.
After attempting to pull this on Josh (neighbor once removed) several times in recent history he sent me this link. http://www.theonion.com/content/node/40547