Personally, I prefer nothing more than a midsummer eve's stroll through Old Fethiye or a shared bottle of wine at a wine house up in the mountains of Kayaköy. Otherwise, nightlife is centered around three main resort areas. The Old Town Bazaar in downtown Fethiye has a good number of bars, restaurants, and shops lining the ancient streets, and shops are open until as late as the money flows in. The promenade along Belcegiz Beach down at Ölüdeniz, with its handful of nightspots and frat-boy mentality, is absent of any real sophistication, and utterly avoidable unless you're staying on or near the Blue Lagoon without any transport of your own.
Hisarönü, the enclave located on the road to Kayaköy, turns into a fun-house festival and pedestrian market on summer evenings. The streets are lined with restaurants and boisterous bars too numerous to mention (a huge draw for the local British expats), and shops selling beach souvenirs that catch your eye under fluorescent lighting. If you're looking for something of a more organized nature, hotels and resorts organize presentations of live music or some contrived line-dance activity. Look for postings around the properties for options.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.