I would recommend trying a Bluetooth speaker and if there is too much interference/distance for a constant signal, connect it via WiFi Airplay.
My choice of gear, which I am using right now with my iPad, is a Bose Soundlink Mini II (add a splash of colour with the protector cover) and if Airplay is needed, couple it with an Airport Express using the 3.5mm connection. It also has a USB connection which can keep the speaker charged.
Bose also has a rugged and colourful Soundlink Color range. The sound is excellent, but not quite as world-beating as the Soundlink Mini.