Bolu Museum is the best stop in town for understanding what sits beneath the modern provincial city. Its archaeology and ethnography displays include material from local excavations, with Roman stonework in the garden and objects from the wider Bolu region inside.
It is not a blockbuster museum, but it rounds out a Bolu visit nicely. Go before or after the lakes, especially if you like small regional museums that explain why a place has mattered longer than its current streets suggest.