Here is what I use: a cute set of portable speakers to my Stiletto listening equipment.
They are Very Small and portable. They have a built in rechargable battery. They fold up to a size smaller than the stiletto itself. The internal battery is supposed to last 7-10 hrs. The sound is a little tinney, but from a small size speaker I would not expect otherwise.
It's small enough, and allows portability. This will be a good alternative to my Boombox as far as portability. I can use this with my portable antenna cable too!
I got mine for about $20. I've seen them on ebay for less than that. They are an OEM version of the LG Portable Stereo Speakers MSP-100.
Before I converted the XM2GO box I used these all the time in Hotel rooms. It also is useful in the rental cars that not have an AUX input for the radio.
I just looked on ebay and I see there are lots to be had
LG Portable Stereo Speakers MSP-100., Cell Phones PDAs, Speakerphones items on eBay.com