Turkish for "cotton castle", Pamukkale is a visually astonishing natural attraction that has drawn visitors to its mineral-rich pools since the Byzantine city of Hierapolis ruled its summit in the 2nd century BC. A truly unique sight, the hillside is covered in brilliant white terraces created by the high levels of calcium carbonate spewed forth by hot springs. Especially from midday to afternoon, you'll encounter plenty of fellow shoe-less (so as not to disturb the make-up of this world heritage site) travelers basking in the pools and taking photos of this fantastic geological attraction.
Most easily reached via public transport (train, bus, dolmuş) from Denizli, there are about a million or so tours that trundle the place, many of which will have English-speaking guides. It could actually be a money saving option if you are looking to combine entrance (about $12 USD), a dip in the travertine pools (about $10 USD), and a visit to Hierapolis (another $10 USD), although you'll be less likely to enjoy the calcite baths at your leisure. Speaking of, a number of structures remain from the two thousand year old city of Hieropolis. The most notable ruins are the 12,000 seat Roman amphitheater, a shrine to Pluto, the Necropolis, and various temples, while the on-site museum is worthwhile for its selection of artifacts and a dip in its pools.
Since its inclusion on the World Heritage list, Pamukkale has come a long way from tourist roads and hotels among its travertine pools, cleaning up its game to keep this natural wonder as pristine as it can be. Don't deny yourself a day trip to see one of Turkey's most highly promoted attractions; you won't be challenging yourself to anything too hard, but that doesn't make Pamukkale any less satisfying.