Now protected within Güllük Dagi National Park, Termessos requires at least half a day (4 or 5 hours, preferably in the morning—start early), and bring a water bottle and snacks or a picnic lunch.
The park opens at 08:30 am in summer. Last entry to the park is at 17:30 (5:30 pm), but you don't have to leave the park until 19:30 (7:30 pm). In winter, the park may open at 08:00 am, and close early if there is bad weather (heavy rain, fog, snow), but otherwise the last entry is often 16:30 (4:30 pm).
Just beyond the park entrance is a restaurant, cafe and snack shop, picnic area, toilets, a small "exhibition room" (museum) of photos of Termessos's ruins, and a forlorn stuffed deer.
Guides wait here on most days (particularly weekends) for those who might want to employ them for a tour of the ruins—not a bad idea if you have a lively interest in archeology, as Termessos is an unrestored site: signs mark the major buildings, and the principal paths are kept mostly clear of brush, but you still get a good Indiana Jones feeling as you poke around in the scrub looking at the fields of tumbled limestone blocks.
At the site parking lot are a few picnic tables, a sign saying "No Picnicking" in Turkish, toilets, a tiny ranger post, a temple and propylaeum dedicated to Emperor Hadrian and the goddess Artemis, and an ancient cistern.
Major sights at the top include the Upper Agora, Odeon-Bouleuterion, Hereon (monumental tomb), Cistern with Five Partitions, Colonnaded Street, and the Corinth Temple.
Seeing Termessos requires climbing over rough rock (some huge stones) and perhaps walking a bit in scrubby, scratchy brush. It is not an easy place to walk. Spraining an ankle or two is easy to accomplish, so be careful.
To reach Termessos from Antalya, leave the city following signs for the Otogar (bus terminal), then signs for Burdur, Korkuteli, Denizli and Mugla. The park entrance is off the Burdur-Korkuteli highway.
You should consider combining your visit to Termessos with one to the Karain Cave, 19 km (12 miles) east of the Termessos archeological site parking lot. The two sites make a good day excursion, and you can be back in Antalya in time for a late lunch and a swim.