Yalova is easy to underrate. From the ferry it can look like a practical Marmara port and not much more, and many travellers pass straight through on the way between Istanbul, Bursa, and Izmir. Stay a little longer and the reason for the stop appears inland: hot springs, wooded hills, waterfalls, and the kind of weekend-escape geography Istanbul uses when it needs air.
The city itself is modern, partly because the 1999 earthquake hit Yalova hard. Do not come looking for an old Ottoman centre. Come for the ferry approach, a waterfront walk, Yürüyen Köşk, and the thermal district at Termal in the hills southwest of town.
Yalova is not a deep destination, and it should not pretend to be one. But if you are already in the eastern Marmara region, it makes a good spare-day escape: soak, walk, eat simply by the water, and use the dolmuş network to reach the green parts beyond the port.