Crutchfield has an assortment of powered bluetooth stereo speakers, some of which are also weatherproof. I have given the Kanto's to my kids and they sound good. Crutchfield has amazing service and return policy.
For me, I love the JBL boombox. It is a single boombox, but is so portable and waterproof it rocks for outside use. I take mine on the boat and around the yard when working outside. I think it sounds amazing, and people who hear it are impressed. I know you can link two or more, but don't know if this is true stereo or not.