You have 3 weeks and said you want to spend 3 nights in Paris and "a few nights" in Croatia. That is going to give you 2 weeks in Italy? If so, I'd suggest that you skip Venice altogether, fly from Paris to Rome, spend at least a week there and then take a train to Florence. Rome is superior to Venice in pretty much every way and I really wouldn't want to smell Venice in late July. I wouldn't spend more than 5 days in Florence. Florence is awesome but it's small and you can comfortably see everything there is to see in 5 days without feeling rushed at all.
I would disagree with this personally. I found Venice(Venezia) to be far more entertaining and interesting than Rome. I won't deny Rome has the famous monuments that everyone should see at least once, but as a city to roam and hang out in I preferred Venice. However if you are talking Italy there are far better places to go that are outside the beaten path. Tuscany is amazing as well, it sounds like you sorta want the main city tourist thing based on what you are asking about, but if you do feel like seeing some nice stuff in Italy:
Cinque Terre:
Cinque Terre - Lonely Planet
Cogne:
Cogne - Wikipedia, the free encyclopedia(my personal favorite, seriously I'd live there)
Also for major Euro stuff that I wouldn't skip, especially if you are already in Europe for 3 weeks. Just stay at Air BnB's, they are cheap and often quite nice.
Barcelona, perhaps I'm biased but it's one of my favorite European cities.
Somewhere in Germany, come on you've got a bunch of awesome places to pick from.
If you are doing France don't just sit in Paris, goto Normady(if you like history), Nice if you want some time on the sea(that whole southern coastline is amazing in my opinion, from France all the way down through Italy), and if you really do want to sit on a damn train to travel(I agree that flying is better unless it's a super short hop), by god stop in Zurich or Geneva...
Lake Como area is always nice if you are dead set on Italy too(it's north of Milan).
You and your wife know your tastes obviously, but I travel there often and I try to balance a few days in the tourist spots(I love history) so I can see all the landmarks, then I try to find the little hidden gem towns that are just so much more fun if you are interested in learning about the real culture and history of an area. Don't be shy about the language barrier, if you are polite and put some effort in I've never seen any major problems. Eat new foods and explore out of your box a little.
I'm not one of those "europe is way better" douchebags, but it does have one major thing the US doesn't and that's a much much longer history. The fact that some of these small towns/buildings, have been there since 1400's or earlier, obviously some buildings say early Roman days, gives the entire atmosphere an interesting little flare if you are into that thing. But fuck if you just wanna go to McDonalds stick to the major cities, they've got that too.