B&W used to make a Rock Solid for outdoor use which was very decent and now very cheap used.
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ipodlounge is a great source of reviews and recommendations for speakers and other accessories. It's iPod-centric but virtually all of the speakers accept output from any mp3 player via the headphone jack. There are two categories you might be interested in on that page, Speakers-All-in-One/Portable and, for larger ones, Speakers-All-in-One/Room-to-Room.