I've done it as a long day trip but I wouldn't suggest that. There's a self-guided walking tour that visits all of the major sites, IIRC about a dozen and a half of them. I remember mostly churches and historic buildings with small museums.

IMO you definitely want to have an evening or two there to see the place at dusk and then at night with the lights. You also want some relaxing time to wander through neighborhoods away from the tourists, to sit it cafes, have a nice dinner, etc.