These lightweight little balls would be great to toss on your key chain or attach to a bag. Although they look a bit like something you would buy for your cat to play with, these little speakers would be extremely convenient.
They come in a few different colors, but I'm afraid all three are slightly girly colors. Likely, the green would be okay with most guys though. I seriously doubt that these produce stellar sound quality, but they would still keep your music going no matter where you are.
Even if you're in a crowded area where no one else wants to hear your music. Even if you've never done that, likely you've been on the receiving end of that one. They come in pink, bright green and purple and are being sold for about $24.