The Sony SRS-XB33 is good for outdoor use. It feels sturdy and durable, with an IP67 rating for dust and water resistance that certifies it to be fully dust-tight and immersible in up to a meter of water for 30 minutes. It's also fairly small and very portable as it's battery operated. It has over 12 hours of continuous playback time, which is excellent. Unfortunately, it doesn't get that loud, and there are some compression artifacts at max volume.