You can get a good idea of what is on the market by searching ebay for
portable ipod speaker battery rechargeable
(Be sure to check "Include description.")
YouTube & Amazon are good sources for reviews of the products you find.
The only home-type portable speaker systems I have experience with are the Logitech S315i, the S715i, and the Philips Fidelio. The Logitech S315i, S715i, and Philips Fidelio come highly recommended (for your application (as I understand it)). Having said that, sound is an objective subject.
How do you feel about the speakers on the iMac (rate them...maybe vs. what you want)?
How big are these rooms?
How loud does it have to be?
Do they have to be pretty or of a certain size?
If interested:
I use a Ryobi 18V L-Ion radio attachment (with headphone input) for listening to portable music players (mostly audiobooks & foreign language instruction) outdoors & in the workshop...something like this:
Factory-Reconditioned Ryobi ZRP740 One+ Radio - Amazon.com
(it uses the same battery & charger as Ryobi tools from the same line).
