There are plenty of situations which warrant music but aren't exactly conducive to real speakers, holidays and day trips foremost among them. Tembo Trunks, a Kickstarter project conceived by two Australian brothers on holiday in Africa, looks to be the perfect acoustic compromise.
The Trunks are collapsible silicone cones that function as amplifiers for regular iPod earbuds. They fold and stack for easy travel, require no power, and can withstand being run over by a car. Basically you toss 'em in your pack and forget about them until you need 'em.
Flaps on the back of the cones secure earbuds in place - they're designed for Apple's stock buds but presumably work with others as well—and amplify the music the buds are emitting to 80dB, somewhere between a regular conversation and the racket made by a lawnmower. Maybe not dance party-level boom, but certainly loud enough for a group of people to enjoy.