For history & culture, Old Town History Walking Tour delivers.
A good guide turns the polished stone into a living story of a clever little republic that out-traded Venice and survived modern bombardment — far more rewarding than wandering blind. The honest catch is quality varies wildly by guide and the cheap 'free' tip-based tours can be superficial; pick a reputable small-group one. The best-value way to actually understand what you're looking at.