I'm looking for a new instrument to play for Morris dancing (I already play Anglo Concertina, Guitar, Mandolin, Baritone Uke and a bit of whistle), and I'd appreciate suggestions as to which instrument to learn. My criteria are as follows...

Must be easily portable
Must be acoustic
Must be able to carry a tune so I can play it solo for the dancers
Must be reasonably loud so the dancers can hear it
Must sound somewhat different to the instruments I can already play
Must be relatively cheap (upto a few hundred pounds/euros/dollars)
Does not have to be "traditional"

Most importantly I'm looking for which instrument that meets these criteria is the simplest & quickest to learn.

All (polite) suggestions will be much appreciated

eto