The cascades at Hierapolis are a must! There are several sites to see there like the museum, Apollos theatre, the climb to St Phillip's site, etc;..there are also archaeological digs happening on site which is cool. The Necropolis will be the highlight of Hierapolis for you too as you will see so many tombs and will enjoy the eerie atmosphere.
go to Pamukkale on a good day Both Hierapolis and Pamukkale are stunning but make sure you go on a good day. For me, unfortunately the weather wasn't good the day I visited. What is disappointing to hear it's that most of the thermal water pools there having been diverted to the greedy 5 star resort hotels in the area.
go to Pamukkale early i agree with the last poster about Pamukkale, though I would also add that you will be disappointed with Pamukkale if you go during the heaps of tourists. Go early in the morning and dont forget to walk thru the ruins and then enjoy the hot water