Montreal a Good Adult Family Trip?

Congratulations! What a memorable way to celebrate! We also went to Quebec City (via train) for my daughter's 40th, since you have enough time, and I highly recommend it. While being there for the Christmas market and holiday ambience was wonderful, it was also so cold and also icy that walking was a challenge in both Montreal and QC.
In September you'll be able to enjoy the many outdoor options. I think it's a perfect choice!

Yes, visit both Montreal and Quebec City for a perfect trip.

evanslaura,
I have plans in hand for a Montreal and Quebec City trip when I can nail my husband down to a month, and when cousins visiting from France give us some dates for their visit to us so we can pick some dates (when they are not here). Aack!

Now that I have vented, here are some of the things in my Montreal list (some are must-sees, some are options):
Notre Dame Basilica, Parc de Mont Royal, the Botanical Gardens, Old Town Montreal, the National Circus Museum (and hopefully a Cirque de Soleil performance), St. Joseph Oratory, Pointe-a-Calliere Museum, the Jean Talon Market,
PLUS! lots of good meals (I have received lots of recs from the forum).

If you decide to spend more time, go to Quebec City also. It has even more things to see and do. I hope for 5 days in each city.
Have a wonderful birthday! Bonne Anniversaire!

Montréal is a fabulous city with lots to do and see, and you should have a marvelous time. Be aware that courtesy of the locals toward Anglophones isn't always the best.

Thank you so much for your comments! Very encouraging.

I believe the Jazz and summer festivals follow one another towards the end of June into the first two weeks of July. First in Montreal, then Quebec City. I've done this. Nobody will be bored.

about 5 days

IMO definately not bored in three full days, five could be a bit of a stretch depending upon what everyone enjoys most. As suggested you could do a couple of days in Quebec City. BTW get the bagels and skip the poutine.

There is definitely plenty to do for five days- especially if you do a day trip to Quebec City (which is very doable but a bit long). I used to live in Montreal and loved every minute (except the winter temperatures). There are a lot of good recommendations here. A few additional thoughts:

-I'm not sure I'd agree that locals aren't courteous to anglophones. Just make a small attempt at French (bonjour, merci, bonne journée, etc) and it will go a long way.
-There will definitely be festivals- when I lived there the film festival was the main one in September but that may have changed
-I definitely agree to try the bagels (Saint-Viateur- they are the best) but can't support advice to skip poutine... you should at least try it, I personally love it
-In September it won't have gotten that cold yet- definitely climb up Mont-Royal. If you want more outdoor experiences you can cycle the Lachine Canal. Verdun is now the cool neighbourhood everyone is visiting- last time I was there I had a very nice Italian sandwich at Bosa.

I'm so encouraged by your responses! And my French isn't bad (took it in high school and college - still have a decent grip on it).

Any suggestions on where to stay would be helpful too. There will be 6 of us but only 4 rooms needed. I usually like hotels but perhaps an airbnb would be good? I'm assuming we will not need a car - my understanding was its very doable without. Thanks again for your responses.

Your days in Montreal can be packed.
Highly recommend Old Montreal, Mount Royal Park. There is the Musee des Beaux Arts ( Museum of Fine Art) on Sherbrooke. I can't recommend any restaurants since it's been decades since I lived there.

September should be wonderful- sunny and pleasantly warm in the low 20s Celsius in early September and gets cooler, of course later on, with less likelyhood of the high humidity of high summer.

I've had a more recent trip to QC last year - it was wonderful. I really enjoyed the Museum of Civilization - they had a special exhibits on Quebec wrestling culture and Quebec hip hop that were so well presented and not something I expected to enjoy and the permanent collection on indigenous artifacts is extensive and very well explained. Walking through the Plains of Abraham park was lovely. Seeing the old lower Town and Champlain district was fascinating too, seeing some of the oldest settler/colonial buildings in Canada. The Ursuline convent museum was interesting, though a bit creepy - young girls boarded there for several years to get what was an acceptable education for girls in the 18/19th century.

Montreal is quite walkable and has a great transit system. It wouldn’t be difficult to spend 5 days there as in addition to museums and the historic areas, there are some lovely neighbourhoods and market areas to explore. When we were there we stayed at the Sofitel on Sherbrooke and really liked the hotel and location. There are other hotels in close proximity. AirBNB is becoming less popular/available in Canadian cities and Montreal had a terrible fire in one a few years ago so a hotel may be your better option. Similarly to France, saying bonjour etc will be appreciated. Montreal isn’s as strident about French as Quebec City can be and it is used to lots of international visitors who likely need to converse in English. Enjoy your visit!

You definitely won't need a car- the métro covers pretty much all the tourist areas and there is also an extensive bus network. I think the vieux-port is a great place to stay. For poutine, La Banquise is now very popular tho I don't think they take bookings. But you can try it pretty much anywhere. Sounds like a really nice trip.

Any suggestions on where to stay would be helpful too.

A couple years ago my family stayed at Hotel Bonaventure. Very reasonably priced and a really nice courtyard with a pool and families of ducks hanging out in the wooded areas. It's located right above the Orange Line station so you don't have to go outside to catch the metro.

Restuarant recommendations:Olive et Gourmando for lunch while you're in Old Town/Notre Dame. L'Express for lunch or dinner (they give you mini pickles on the table instead of bread). Schwartz's Deli is as close as you'll get to an old school NYC deli.

Thanks for that new infor - my maiden name is Schwartz so clearly I was destined to go to that deli lol. Also appreciate the suggestions on the hotels and restaurants. This is great- a bit nervous about traveling to Canada now in this political environment so hope there are no unexpected problems.

I expect you will be fine bearing in mind to not joke about any of the current issues or wearing specific clothing with a particular 4 letter acronym:) We are welcoming Americans who are interested in visiting and appreciate our country. Enjoy.