You may want to take a look at the Audivox Acoustic Research outdoor speakers. The model AW825 is made for outdoors settings (though not waterproof), has a nice mission-style design and gets good reviews on Amazon. You’ll need two sets at $75 each if you want stereo sound, though they have a left-right switch so you’ll get true stereo. I haven’t heard these speakers personally, and I wouldn’t expect great sound, but should be fine for playing out on the patio. Here is the link.
Another option is the Audio Unlimited speaker set at $89 (for two speakers). They are water resistant, can be wall mounted and also get decent reviews, though seem as the may be bit quirkier to work with. Here is the link.
My intuition is that you may get better sound from the Acoustic Research. Whether you like the design of the AR or the Audio Unlimited better may depend on whether you intend to mount them or keep them portable. I don’t know of any other solutions without a significant step up in cost.
Good luck!