For the ultimate in portable monophony try the Stylophone! Seriously, the new ones have a mini-jack input for an mp3 player, so I've been creating drones by way of backing tracks and using the mp3 player to play them back through the stylophone speaker, and using the stylophone itself to accompany the voice in unison. I might add that so far this has been an entirely domestic diversion, though I dare say I'll be taking it out into singarounds soon.

For a sample of how this actually sounds check out my Stylophony Number One at http://www.myspace.com/dh7haha.