Saklikent (Hidden City) Gorge is the second-largest (20 km-long) gorge in Europe, the longest and deepest gorge in Turkey, about a 40 minutes’ drive from Fethiye. It is a spectacular place, with sculpted walls soaring high above. Four km. of the gorge is walkable after April when most of the snow from the Taurus Mountains has melted and passed through the gorge on its way to the Xanthos River. Summer is the best time to visit as the canyon is deliciously cool and shady with cold water.
Tour operators in the coastal resort towns offer tours to Saklikent Gorge, often in conjunction with a visit to Tlos and the very lovely and relaxing Yakapark Restaurant (also Jeep safaris from Fethiye, I think). Some tour operators also offer canyoning trips to the gorge.