Last time I visited Italy, I had a 30D, 10-22, 50 and 70-200. Most of my photos were taken with the 10-22 and the 50mm. The 70-200 didn't come out of the bag too often.
24mm at the wide end is probably "ok". But if you've got a 16-35, I can't see why you wouldn't take it. You'll have plenty of opportunitites to use it at the wider end. Plus, it would be a good choice for indoor photos. I often used the 10-22 at the 10mm end and often wished it was wider. I don't have a fish-eye so can't comment on this.
While the 16-35 will produce "slanted walls" etc, if distracting, this can fixed later in software. But a tilt shift lens might produce a better result. (When you try to fix this via software, you lose part of the photo which makes framing difficult).
I took a good tripod with me on that trip. But I was travelling with a baby and didn't get a lot of chances to go out at night to use it. If I was doing the same trip now, I'd probably take a tripod again and try to get more dawn, dusk and evening shots. But I agree, you can get away without taking one and buy one if needed. Maybe take a gorillapod?