I use one of these
but not the plastic version, rather a home made wooden article done up to look like piano keys.
Its a simple way to create that Zydeco rub-board sound without having a corrugated iron vest round ya neck :-)
Lark in the Morning have a huge selection of weird and wunnerful instruments.
I also carry around a cajun triangle, two coconut halves, a shaky corn-cob, a wee tambourine, a shaky banana and some plastic knuckle dusters with bells on.
A true artist, thats wot I is, mate.