Help me plan Anniversary Trip

You will not have to rent a car for that itinerary.
I count 8 nights, not including your arrival and departure dates.
Would do it in this order:
Venice 3 nights
Florence 2 nights
Rome 3 nights
You don't do anything in Milan except fly out from there.

Thanks for the advice, Kent! Do you think trying to get to Parma/Bologna would be spreading ourselves too thin? I'm not familiar with the rail service. I'm thinking if it doesn't cost too much extra $, that on the way from Rome to Milan, we could stop off in those cities for the day?

If you can do it during the travel day, why not.

Something you don't want to do, with your 8 nights constraint, is to try and do any day trips from Florence. 2 nights in Florence is rock bottom as it is.

April 3-6 Venice (3 nts)
http://www.hotelalcodega.com/
April 6-9 Rome (3 nts)
http://www.hotelsantamariatrastevere.it/
April 9-11 Florence (2 nts)
http://www.hoteldeimacchiaioli.com/
April 11-12 Milan (1 nt)
http://hotelberna.com/en/

I imagine you have a morning flight to the USA from Milan so will need to stay there the last night for sure.
Milan is worth at least a half day (get an early train from Florence) for the duomo etc.
Book train tickets 3 months out with trenitalia - don't buy a rail pass.

You count nights, but don't count your arrival night.
2 nights in a destination = only 1 full day of sight-seeing.

Thank you guys! I appreciate the advice and we are super excited. We just ordered the RS Italy 2015 guide book. We used his books in London and Paris, but we didn't travel all over as we are planning in Italy. Will the Italy guide book be enough, or would you recommend getting the separate Venice/Florence/Rome guide books? We love his detail, but we also rather not lug around 3 guidebooks.

Italy guidebook will be sufficient for the length of your stay in each city, you can save the $70.

Milano may surprise you. There are some museums, the Duomo (you can climb or take the elevator to the top, or go to the rooftop bar of La Rinascente across the way for a closeup view of the roof and an anniversary cocktail). Ristorante Settembrini 18 is a couple of streets away from Hotel Berna.

Just a quick mention - if you have an iPad (or maybe other devices as well) you can get the RS guides on that - that's what I did for our Rome guidebook last year - didn't want to have to lug around a book...and since I was taking my iPad anyways...you can highlight pages, bookmark, make notes...and if you are connected to wifi, the www links in the book will take you right to the websites...I also did my California guide this way...

I did the same idea last November we just used the trains. We booked all tickets ahead of time, we loved Venice, Rome and Florence.
I also suggested skipping Milan

Since you don't have a lot of time I might suggest skipping Rome this trip and concentrate on the area near Venice and Florence... you can add some Tuscan villages which might be a nice addition to the cities, plus Tuscany is very romantic... a visit to a few vineyards, some roadside picnics, etc. Also, you could then spend a day in Milan.

Would I need a vehicle if we decided to skip Rome and visit some Tuscany villages? I'm not sure about skipping Rome...after going to Paris and being amazed at the history there...I can't imagine not seeing the Coliseum!

Exactly, as a first time visitor to Italy you definitely can't leave out Rome.

Apr 3 - land in Venice (2 nights), easy sightseeing with jetlag
Apr 4 - Venice
Apr 5 - train to Florence (2 nights) (4-5 hours: pack, check out, get to train station, 2 hrs on the train, get to hotel check in); 1/2 day sightseeing
Apr 6 - Florence
Apr 7 - Florence sightseeing, evening train to Rome (4 hours with 1.5 on the train - take a picnic dinner, this maximizes sightseeing time)
Apr 8 - Rome
Apr 9 - Rome
Apr 10 - Rome
Apr 11 - morning train to Milan (5 hours or more, with 3-3.5 on the train)
Apr 12 - fly home

It's a feasible plan to fit in all the cities. The above is just a one itinerary to see if it works. How much time you have to sightseeing on your travel days depends on how early you get up and how quickly you get organized. You need to allow time to get to the train station and find your train. Usually 15 minutes is enough, but allow for possible traffic in Rome and Milan. Unless you are very close to the train station in Venice, it can take up to an hour to get there.

What time is your flight from Milan? You'll probably want (need?) to be at the airport 3 hours before that and the ride (bus or train) is close to an hour. Buses and trains start running around 5 a.m.

You can stop in a lot of places on your way to Milan, you just would need time and you look to have too little of it. Fast trains take a little more than three hours (sometimes even less than three hours) between Rome and Milan. This is incredibly fast travelling (speed tops at 180 mph) and much faster than flying once you take into account traveling to/from airports, time spent at checkin and security, etc.

If you begin stopping at smaller cities along the route, you will be committing to much slower trains that will eat your time. Just stopping in Parma along the way would likely add a couple of hours of travel time plus the time actually spent in the city. If you had more time I would say, absolutely do it - sometimes I have timed my car travels just to spend a couple of extra hours in Parma to have lunch. - But if you are scarce on time you cannot afford it.

Any way you can extend your trip? I'm sure you'll love Italy and 2 weeks would be great if you can swing it. We have found with AA milage award tickets that there is no charge for changing dates, as long as your enty and exit cities are the same. Also, we have never booked our Trentitalia tickets before hand. It shouln't be too crowded in April and buying the tickets from the ticket machines at the train station is easy and cheaper. We usually buy our tickets for leaving a city when we arrive in a city. Rick's Italy book has a good description on how to use the machines.

Unfortunately, we are unable to extend the trip. My 8 year old is already mad at us for leaving him behind this long!

Thank you for the suggestions. I appreciate your input!

Tuscany is best seen with a car should you decide to do that. If you did skip Rome you can save it for when you bring you son:). I went to Italy 4 times before I made it to Rome, and didn't regret it at all. If it's a priority then go.

Rome is a very special place. I'd try to stay near Piazza Navona/Pantheon if at all possible. It is a wonderful area for day and night walks with many quieter streets, away from the busier intense main streets. I would definitely spend more time in Rome over Venice. Venice, is also special but much more touristy. We especially liked the Peggy Guggenheim museum there.