Ordering Earl Grey tea from the computer, then smacking himself on the forehead and saying "I could have had a V-8!"
Yelling "Punchbuggy!" and hitting Riker's arm whenever he sees a shuttlecraft
Screwing around in the holodeck when he ought to be on the bridge
Spotlighting unsuspecting crewmembers with the glare from his forehead
Lecturing everybody on why it's rude to fire the phasers at other life-forms
Sending crank subspace messages to Starfleet Command asking if Dick Hertz is there
Asking Beverly Crusher to come to his quarters so he can show her "a REAL Picard Manoeuvre"
Ticking off Romulan commanders during tense confrontations in the Neutral Zone by asking "Are those Bugle Boy jeans you're wearing?"
Telling crewmembers in menacing, Dirty Harry voice, "Go ahead, Make it so"
Putting banana peels on the transporter pads just before an away team beams back up
Skeet shooting the shuttlecraft
Plugging Nintendo cartridges into Data
Ordering Pizza from Domino's then going 30 minutes into the future just to piss them off (haha, free pizza!)
Reprogramming the computer to play the theme to Jeopardy during self-destruct sequence
Tribble sex!