For outdoor use, you probably want powered "PA speakers" rather than computer speakers or monitors. But, you probably won't be able to get a pair in your price range. You might have to settle for mono. Here are some examples. (Some of these you might not consider "portable".)Or for about twice your budget, you can get a little portable PA system something like this. (These usually are not stereo either, although they often have two speakers.)Note that most "studio monitors" don't have grills over the drivers which can make toting them around risky. PA speakers/monitors tend to be a bit more rugged.
So far, what comes to my mind is the M-Audio AV40s, AudioEngine A2, and Samson MediaOne 3A. (m-audio av40 reviews tend to show many heat/reliability complaints tho)