I think this is one of the nicest things about Hop-On Hop-Off tours. You can actually slow down when something catches your eye instead of rushing through just because the group has to move on. I’d probably use it for a first visit to a city, then spend more time at the places I end up liking. I came across afkspin while browsing, but the flexibility of these bus tours is what stood out to me here. The only thing I’d definitely check beforehand is the last bus time, because missing it could make the freedom a little less useful.