You will be tripping over good places to eat. Especially in Rome. You won't understand it until you get there.
4Leoni in Florence is really nice, reasonable enough if your looking for the 1.3 kilogram Florentine steak. I would make a reservation for 4Lenoni. You email them and they get back to you. We ate there a few weeks ago. Superb. Not over the top, dinner for three with several main courses, the big steak for two, a couple starters, several side additions, a liter and a half of wine, water and soda came out at 145 euro and we had A LOT of food. Obscene really - but hey we are on vacation!
In Venice our favorite place was in Lido at Parco delle Rose (closed Tuesdays). Take the water bus to Lido, stroll down the main street - its on the left just past the bumper car arcade. Really nice fish as well. Reasonable prices (the fixed menu is great - lots to choose from). This was just so relaxing and comfortable (outdoor dining) in a huge garden. I can't remember what we paid, I think fixed menu was around 20/pp for starter/app, 1st course pasta dish, 2nd course fish dish, salad and desert (very nice),a liter of wine plus a nice cutlet, fries and sodas for the kid total was around 60 Euro at the end I guess. That said you can have a pizza for 8~ Euro there, or a lasagna or pasta for about the same or less. We could easily of halved our food expenditures but why??
Rome - actually the small hole in the wall street food and small places were great.
Sorrento - Inn Bufalito - a little hard to find. Nice food.
Above are the ones we really enjoyed.
Have a super trip!