Uilleann bagpipes. Instead of blowing to fill the bag with air, you pump a bellows with your right arm, and squeeze the bag with your left.
You might ask your self why uilleann over Highland Bagpipes or the like. The answer is simple: if you're not blowing into a blowstick your mouth is free for singing and, more importantly, drinking.

The glass harmonica would be awesome for no other reason than the hilarity around trying to move the instrument. People's reaction to the crazy sound would be fun too. Alternately, one could use Franklin's armonica.