Google has promised standalone music playback over Bluetooth as part of Android Wear for some time, and it appears with Wear version 4.4W2 we're finally getting it, at least if you use Play Music (you're also getting new playback controls). If you're using the latest Play Music APK on your Android phone and have the Android 4.4W2 update on your smartwatch, you can now download your pinned music from your smartphone to your watch.
From there, you can play songs that have downloaded directly from the watch to a connected Bluetooth audio device, like some headphones. Just tap on the watchface and swipe up, I'm not sure if there's a specific voice command for this activity yet. Go down to "start" and you'll see Play Music. Then just pick your album - select albums by swiping up and down, then swipe right for shuffle mode, and again for a list of tracks in the album.
I'm not sure if this works with playlists, but I assume so if you have them pinned. It's not clear what the upper storage limit is, and I don't exactly have time to test it just at the moment.
All in all, another nifty new Wear feature that adds to music experience - something smartwatches are undoubtedly good for.