If you take your party music seriously, then the PartyBox 100 Portable 160W Wireless Speaker with Built-In Light Show from JBL could be an ideal choice for your backyard bash. Nearly 2' tall, this speaker blasts 160W of audio out of two 5.25" woofers and two 2.25" tweeters. If that's not enough, you can get another PartyBox 100 (not included) and link them together via Bluetooth for true wireless stereo sound.
There are plenty of options for getting music to play through the PartyBox 100. You can use a USB drive with a custom playlist, you can stream from a Bluetooth mobile device, or you can plug in components via 3.5mm AUX. If you've got some talented guests, you can shake things up by plugging in an (optional) microphone to sing along, or your guitar to show off your shredding skills. Whatever you play, you can do it all night long with up to 12 hours of playback on a full charge, or go even longer by plugging in the PartyBox 100 with the included AC power cable.