is 2 full days enough for Venice ?

2 days is better than 1 day, 3 days is better than 2 days, etc. You only have so much time. Sounds like you want to spend some extended time in Rome. Adding time in Venice takes away time from Rome. All in all, if that's all you have time for, so be it.

Spending a day to visit your friends and then staying in Venice is definitely better than doing day trips from Padova.

Personally, I would drop the days at Rome to four, stay one night with your wife’s friend at Padova and then head to Venice. She probably has many interesting ideas to show you around Padova - the nice piazzas, the Scrovegni Chapel, villas in the countryside, etc. and sitting in the piazzas for a meal with a local would be wonderful. Just don’t commute from Padova to Venice as you would miss the special times of day in Venice that you mentioned. Enjoy your trip!

I think your plans are good. You can see the main sights in Venice in two days.

Don't cut Rome, I have been there twice, spending a total of 12 days and still didn't see it all.

Good plan: 5 nights, 4 nights, 3 nights. Good instinct to stay in Venice instead of Padova. You'll want to go back, of course, but we all do!

I agree with Jean - drop a day from Rome and add it to Padova/Venice. We wish we had done so on our trip last year.

A good bit depends on what you want to see in Venice. Going inside museums (like the Guggenheim) takes a lot more time than just wandering past them, soaking in the atmosphere. Getting to Murano (not that it is essential) takes a comparatively long time. I like museums and like glass, so I would need more than two days. You may be reasonably satisfied with that much time.

I want to reassure you about Padua, though. It's a very interesting place. I very much enjoyed being there for a couple of days.

IMO too much time in Roma- sorry Roma! There is so much to see & do all over Italy that I would do 3 days max in Roma- the things you will do there are relatively central to each other- the Trevi fountain (wow in such a tiny spot!) the Vatican archives- (only so much gold encrusted hooha before I’m bleary) the sixteen chapel- totally worth it as is the Pieta or (any Michaelangelo sculpture anywhere!) the forum- where democracy was formulated! Great park too! This is the only city that you will most likely face beggars, extremely crafty pickpockets, and the like. Please don’t leave before you have pasta Cachio y pepe- it’s sublime in its simplicity and totally Roman. And then escape to soak up the countryside and art of Tuscany - Padua is lovely & you will feel like a local with your friend, then run to Venezia for 3 days, one in Murano for glass, a longer ride to Burano and Gatto Nero restaurant will reward you with colorful scenes to play in your head for years.

Drop a day in Rome and add it to Venice, and stay in Venice
Regards

I spent three nights and two days in Venice during what turned out to be (unexpectedly) high tide season. I think that's a sufficient introduction to that area. You may want to return at some point, but it was a wonderful first time amount for me.

Thanks to all for all those insights, very useful ! 

So the general idea seems to be that 2 days in Venice is not enough. Thanks, once again, for all the ideas to add a day there. Some things to consider: 

1) We wont have a late dinner with friends, they are working (but plan to take one day off for us) and they have young kids. 
2) 5 nights for Rome was mainly to get rid of jet lag. 
3) I know Florence is a powerhouse but we have no intention to see more than one museum (Uffizi or accademia) and even there it's not mandatory

What do you think of that:
July 8 arrival at rome at 11:20. Day lost because of jetlag. Get the appartement, have dinner. sleep.
july 9 Rome at 50% shape...
july 10 Rome at 100% shape (St peter ? Ostia Antica ? Colesseum ?) 
July 11 Rome
July 12 Rome
July 13 departure for Florence, arrival around noon, lunch get the appartement, walking and dinner
july 14 Daytrip to Sienna (if all tell us that we must more see Florence instead of daytrip we will consider it) 
july 15 Florence (maybe one museum)
July 16 Departure around 9:00 for Padua. Leave Padua for Venice at 17:00
July 17 Venice
July 18 venice (daytrip to Burano ?)
July 19 Venice
July 20 Departure for Montreal at 13:40

The thing about Venice is that we have read that it's hard to find good/authentic/ not overpriced food. 

I think that time allocation is fine. Almost all of us have less time than we want is some places. That's just life for any traveler with limited vacation time (or Schengen time).

Even decades ago, I found food (typically marvelous in Italy) quite a challenge in Venice. I've seen posts on this forum with some suggestions, though. Generally, the odds are more in your favor if you walk away from the tourist-clogged areas. Keep your eyes on the Italy forum in the next few months, and try the search function.

I've never been a great fan of TripAdvisor's restaurant ratings, but they might be useful as part of your research, especially if you use Google Maps to verify that the interesting ones aren't within spitting distance of places like the Rialto Bridge.

I certainly agree to add the 1 day to Venice. This is our 6th trip there in May but sad only for 1.5 days. However, I will certainly be on the back streets to look for a more authentic restaurant. Just love that city. You'll either love it or hate it, but it has to be one of the most unique places in the world.
Enjoy.

WAIT!!! Forget about jet lag. I'm assuming that your flight will arrive sometime in the morning. On my very first trip overseas, I roped my son into going with me and he told me we'd be tired after our flight but he had found that the best way to deal with it was to get to the hotel, get unpacked (or leave luggage if room isn't ready) and then go out wandering. If it's a city with those hop-on/hop-off buses, take a tour of the city. Venice of course doesn't have Venice but I think I remember seeing mention of hop-on/hop-off water buses. Anyway, the goal is to stay up til at least 9:30 or 10:00 p.m. because after a full night's sleep, you are ready for morning and it will BE morning. Maybe use that first day to just learn how to get around Venice - it's very different from a city with actual streets. Or decide on a museum you want to visit (I wouldn't recommend the Doge's Palace - you really want to be rested up for that!) and while away some time there. Have a nice lunch near some water and watch the world go by. For me this method has always worked like a charm. I wake up the next morning, have a nice breakfast and am ready to go.

Have a great trip and don't shortchange Rome unless Venice appeals to you more. Many, many years ago when we were stationed in Germany we spent 9 days in Rome and every day was filled with wonder, excitement, and just sheer pleasure! And we sure hadn't seen it all. However, now in retirement, when I first decided to start traveling overseas, I ruled out Rome because I knew that I could never recapture the excitement of that trip!

Thanks to all once again for the follow up !

Marylynsim: I wont arrive in Venice but in Rome. I know how to fight jetllag but being our fifth european trip on the last 10 years we know how we feel those days.

ACraven: Thanks ! A always take tripadvisor review with a pinch of salt., but the idea of combine them with google map is a good one.

Good to trim Roma- if you really love it it’s easy to return-
Re: Sienna- it’s an interesting good example of a Tuscan town- ( I like S Giminiano better, it’s smaller) the thing about getting out of Florence isn’t so much about the town you visit, it’s about traveling the countryside! And you can’t mess that up even if you try!
Re: Uffizi - that’s the one you don’t want to miss- get your ticket ahead of time & you will bypass the inevitable line- and you can do that & see the David in the same day- you want to see David in the original live because that rock is mid breathe- it’s crazy! The rest of the academia imo is meh after David, which is spectacular. I’d pay again to see it. And the Uffizi. Gelato in between. Of course!

Re Venice food- go for the appetizers and primi- must have risotto-
Meat is not the thing there!
Soups, pastas, risottos.
Ask locals where they eat.

maryjanezapp, thanks for those insights !

I've never think of trimming Rome. I think 5 nights there because it's our arrival place, needs to be 5 nights.

If I keep 3 nights for Florence i wont have muuch time to see countryside beside only one daytrip. We still haven't decide where stay 4 night...Florence or Venice.

It doesn’t sound like you are super into art but the Scrovegni Chapel in Padova is one of Italy’s most important masterpieces! Before entering, visitors dehumidify while watching a very good video that explains why the chapel is so important.

My tip for eating in Venice: it’s very important to make dinner reservations! Otherwise you will find yourself wandering around starving without any options. Do your research about good restaurants ahead of time. Most restaurants in the center of Venice are overpriced and mediocre. Also, find a grocery store- there are a few near the Rialto bridge- and make sandwiches for lunch (find a quiet corner to eat discreetly). This will save you a lot of money and time.