Florence is arguably the best possible city for a honeymoon. This is for two reasons:
1. There are really nice hotels with wonderful rooms to stay in.
2. There isn't much to do, so you won't really be tempted to leave those rooms.
That may sound like a condemnation, but it really isn't. Florence (Firenze in Italian) is a beautiful city with sights to see, food to eat, and some wonderful art galleries; it's just that you shouldn't feel the need to see any of it if you simply want to laze around (or perhaps not really "laze"...) in bed. It's small enough to wander around randomly and see something interesting, and large enough to support several days of such wandering. There is a gelateria on every corner, and three restaurants between each of them; there are stone buildings dating back a thousand years, and galleries showing art nearly as old. The home of Dante (yes, the author of Divine Comedy) is here, as well.
Our hotel there was very nice. It was originally a "palace" built by a wealthy merchant, which has been divided into about a dozen suites (and a huge amount of space whose purpose I haven't yet divined). We had two bedrooms, a living room, a kitchen and a bathroom, with breakfast every morning, right in the city center. It was extremely nice, although very expensive—€300 a night, apparently—but just the view from the rooftop terrace was worth it. (We don't usually stay so extravagently—the parents were muttering about the cost the whole time.)
Rome, on the other hand, has the feeling of a place that had its heyday 1800 years ago and has been suffering through a slow, lingering death ever since. Even the manhole covers have SPQR on them, with graffiti layered on top. But that's only an impression from half a day—we'll see if a few more days changes it.