Can you give some size and $ limits?
On the slightly bigger side, you have the "portable" ipod docks. There is a veritable deluge of options to choose from in that area.
For USB speakers, the Logitech V20 are decent and you can frequently find them on sale for $40-50. They come with their own semi-hard case too, which is great for transport.
On the super cheap end, I picked up these $10 flat panel computer speakers from Walmart for my mom a while ago. I forget the name, but each speaker is about the size of a calculator and magnetically clip together. Plugs into both the headphone jack and usb (for power; or it can use batteries). It's good sound for $10, and small/sturdy enough to fit the whole thing in a pencil case.
edit: found a similar one for $30,
http://walmart.ca/Canada-Electronic....Id=384&tabId=1
but my local store carries a knockoff brand for $10
I did compare the two before, and the expensive did sound better.