You could use a small FM transmitter that plugs into the Dock Connector (like the Griffin iTrip) and broadcast it to a small handheld radio in your pocket.
The Cube device above looks really thick and bulky for "wristwatch" use. The iTrip is thinner and smaller overall than the Cube above (yet still a tiny bit wider than the 6Gen nano, I believe), and would work with any FM radio, so if you want to have an inpromptu house party, just place the nano + iTrip near a home stereo and tune in.
EDIT: I'd like to see Apple or Belkin or Griffin make a Dock Connector mini-speaker; it could be tiny, like the speaker built into the 5th gen nano, but powered by the Dock Connector with a small, unobtrusive profile. The headphone jack does not generate enough power to move speakers larger than headphone speakers, so that's IMO not a good route for a solution.