Carrying a mobile speaker for playing loud music while chilling outdoors isn't all that difficult. That is, provided you've got a bag with enough space to cram one in (unless you're fine carrying the Trash Amp in hand like a can of Coke the whole time). If you don't, then you need something even smaller. Something like the wallet-size Card Speaker.
While not really the same size as a card (it's thicker), it is small enough to slip into any of your pants' pockets, ready to be taken out at a whim. Heck, it might even fit in a large wallet if you're rocking one of those. The product page claims it "sounds as good as it looks." Well, it looks a miniature speaker without much power, so I'm hoping the sound is actually a little better.
The Card Speaker measures 3.3 x 2.2 x 0.3 inches, making it a convenient alternative to your smartphone's tinny speaker when playing music on the road (just plug the 3.5mm connector and go). It features a metal construction, with either a matte gold, stainless or shiny black finish.
You can think of it like a really small boombox, probably sans the "booming" part. It has a ratings input of 0.7 W (8ohms/1.0KHz), with a USB-rechargeable battery offering up to 5 hours of playback on a single charge.
If your mobile gadgets aren't cutting it for quality audio playback (you need your Dance Megamix medley fix, after all), you might want to consider getting this dedicated Card Speaker instead. It's available now, priced at $74.