I just use those large cotton squares (or sometimes rectangles) with (or without) fringes on either end that you can buy anywhere. They come in tonnes of colours and patterns but my favorites are the batik'd ones in bright tropical colours. They can be wrapped around your waist to become a floor-length, bottom-only coverup, or can be tied in the front above your breasts to become a thigh-length, restaurant-friendly coverup.
Here's a link to an image of different ways to tie a pareo:
http://www.idcw.org.uk/Graphic/PAREO.jpgMarci (of Howard & Marci)