If you're in a hot, humid area like the SE U.S,, there's not much you can do. If you have solar heat, running it at night so that the panels radiate heat into the (slightly) cooler night air will help some. Likewise, some kind of fountain (as Philbo suggested) will produce some evaporative cooling, but with high humidity there'll be very limited benefit.

If you're in an arid climate a fountain will help quite a bit.