Psalteries (the plucked kind) and harps get my vote. They're authentic, they're portable, you can sing to them or just play melodies, and you don't have to be an advanced player to bring out nice sounds on them. They have naturally pretty voices which go fine with simple music and beginner's abilities. They're fairly hardy in the outdoors too.

Barefoot guest, if you want to bring an instrument and sing, you should bring and instrument and sing. You sound like the sort of person ren fairs need more of. (Still wish you'd put on some shoes though.) I hope you'll come back and tell us what you did there, what instrument you chose, if you liked playing it and if you're going to keep on with it. Good luck to you.