I asked this earlier this year. If you use the search function you'll find lots of great advice.
We went the last week of Aug with three children. The Botanical Garden was terrific and the kids loved the Biodome. You can buy combo tickets for both at a discount and there is a shuttle bus between the two places. This location is the former Olympic sight and we returned to ride the cable car to the top of the "fin" of the arena.
Another site we stumbled on was St. Joseph's. It is the huge copper dome seen from everywhere. Many pilgrims come to pray and the building is open for sight seeing. I found the interior surprisingly modern.
We used the metro almost exclusively, and the one time we didn't we were very sorry. It is faster by train than by car.
The hike up Mt. Royal is wonderful, an art exhibit was there amongst the woods. We enjoyed the scavenger hunt to find all the works. The view at the top is lovely...
Old Montreal is a bit touristy but we loved it. We walked around the village and a block or so off is a neighborhood consisting of tons of restaurants. We had a hard time deciding!
We missed many of the historical buildings, like the Notre Dame etc. The kids had just so much tolerance for that and we only had 4 days. For them the trip was memorable for the little things..The 7 year old loved the subway rides, the 10 year old loved the Orange Julep place, a HUGE orange ball of a building right out of the 50'-60's novelties. Food was awful, but we were in the hotel across the street so we never got lost! The oldest used his French to read the ads and placards everywhere we went.
I would love to go back...
If you are driving, it was worth the trip to stop off in Shelburne VT for the Shelburne Museum, something there for everyone...
I found that changing money was a better deal at the merchant. The hotel's rate was not as good. Most places will take US dollars but will give change in CN.
Thanks to Jodi's advice, we brought all of our birth certificates and border crossing was fine. The 7 year old askrd the guard if the tooth fairy came in Canada (he had lost his just that morning and was worried). Her reply, "Yes, but she gives in CN dollars so you get less money!"
Have a great trip!