you could use the creative TravelSound 200($50), regular version with AC adapter($70) or i300($80) all use thier own power source(AA batteries)so you won't have to worry about power draining problems and all are very portable.
if your on a budget though you go with something like the monster iSpeakers($30).
the iSpeakers and the TravelSound with AC adapter are both on apple.com and the creatives are on creative.com under the speaker systems.