Lake Abant is Bolu's classic escape: a clear mountain lake at about 1,350 metres, with wooded slopes, picnic spots, and a road and path around the shore. It is pretty in a straightforward way, which is exactly the point.
You can walk, cycle, ride horses, or just sit with tea and watch the weather move across the water. It is about 35 kilometres from Bolu, so it works as an easy half-day, though weekends can feel busy with visitors from Istanbul and Ankara.