Aizanoi is the strongest reason to give Kütahya a full day. The Roman city lies by Çavdarhisar, about 60 km southwest of Kütahya, and its Temple of Zeus is one of the best preserved Roman temples in Anatolia.
The site is spread through and around the modern village, with a theatre-stadium complex, bridges, baths, and market remains as well as the temple. It feels less packaged than Turkey's most famous ruins, which means you need a little imagination, but the reward is a serious ancient city without the big-tour-bus pressure.