I've got a couple of tweakers (search for it on Amazon). The general design is very portable, quickly charged, and adequate for reasonably quite spaces. Sound quality (on the ones I have) is better than the iPad, but you're not going to care for music on it. You can find them on sale for very cheap.
For a bit more volume I have an iHome iDM12. I set it behind the iPad and it does a fair job for watching movies, even with the AC on in the background. Though it's not loud enough for my rather noisy truck. Sound quality is ok for non-audiophile me, but of course this small a speaker doesn't have much base. It's also bluetooth, so no cables to fiddle with (until you have to charge it). The included stand (and speaker cover) they show only works without an iPad case, in case you're tempted to make a decision based on the pictures.
What I'd really like is one of those Jawbone JamBox speakers; but not quite badly enough to fork over the dough. My toy budget is being conserved for the next iPhone. But if you're after decent sound in a small, rechargeable, bluetooth package; they're the current standard to beat. They come in two sizes.