Although most of these kind of excursions are typically family type of day-outs, I must admit that water here can be pretty rough. We really enjoyed rafting in this 9°C water (ice cold in turkish norms) in a forest - like area.
When staying in Antalya area around august to september, there is nothing much else you can do but spending your time around the hotel pool or the beach. We, from our part, preferred the beach as it was still found to be the coolest place. Many water-sports are available.