I visited both locations of this WHS in Spring 2021. Of the two, Letoon seems to be the less visited. The original highlights of these two locations are now to be found mainly in museums: a whole hall in the British Museum for Xanthos and the Lycian trilingual (Lycian, Aramaic and Ancient Greek) stele in the Fethiye Museum.

At the Archaeological Site of Letoon, upon close inspection, you'll be able to spot some worthwhile details among the original remains (think Greek WHS remains) of what was once one of the most important religious centres in the region. The site is located south of the village of Kumluova and lies approximately four kilometres south of Xanthos along the Xanthos River. The foundations of three temples are clearly visible, with the Temple of Leto in the best condition, and an almost intact theatre. 

At the Archaeological Site of Xanthos, most highlights are replicas such as the impressive Harpy Tomb reliefs (which are in the British Museum). However there are still some other original highlights such as the amphitheatre, the Lycian Dancers' sarcophagus, and the Xanthian Obelisk, a trilingual monolith recording an older Anatolian language conventionally called Milyan, Lycian and Greek. For me, the trilingual stelae alone at both sites were absolute highlights and worth the visit. While Letoon was fenced off, Xanthos (at least most of it) wasn't and having arrived very early in the morning I could visit at leisure for free well ahead of the official opening time.