Pamukkale - Hierapolis
The ancient site of Hierapolis was a Roman spa. "Hierapolis, foremost land of broad Asia, mistress of the Nymphs, adorned with streams of water and all beauty." Romans came to Hierapolis to bathe in the hot springs for their healing properties, and worship at the Temple of Apollo.
Pamukkale travertine terraces -
Centuries later, the natural springs at Pamukkale (which means "cotton castle" in Turkish) are still going. Calcium carbonate is dissolved in the hot water that seeps up to the surface, as it cools, it solidifies into new shapes. The result is a glorious hillside, where turquoise waters flow down white chalk terraces like a layered castle.
Boardwalks go around the terraces so you can view them close up, and even wade in a few shallow pools on the path. If you want a real soak in the thermal waters, go to the Pamukkule Thermal Baths (Sacred Pool), a clear warm mineral pool with pieces of ancient stone columns on the bottom (lockers are available, but bring towels).
Hierapolis -
Roman main street - Once you enter the ruins, the first thing kids will see is a wide main street, which runs north-south, the length of the ruins. Head left towards the North City Gate, into the Colonnaded Street paved with stones, until you reach the big triple arch, Domitian's Gate.
Necropolis - Continue a little farther to the north Necropolis, where people were buried - there's over a thousand tombs here. Looted long ago by tomb robbers (despite inscriptions warning them off), it's fun to wander among the tombs, all different shapes and sizes, and some are roomy enough you can go inside.
Theater - Back the other direction, go east past the Temple of Apollo and the hissing cave of the Plutonium (where originally people could ask questions of the oracle of Pluto, now covered with a grate) to the huge Roman theater. Plays and water shows were performed in the large open amphitheater that could hold an audience of 10,000 or more. The theater is in good shape as it's been restored, and performances are produced here in the summer.