Termal is the main reason to linger in Yalova. The geothermal springs rise in the hills southwest of the city, feeding public hammams, pools, and spa hotels in a district that has been drawing bathers for generations.
The built-up valley is not especially lovely at first, but the setting improves as you move west into the forested slopes. Come for the water rather than architecture, and avoid treating it as a quick photo stop. The point is to soak.