Quite frankly, in Montreal in August it’s hot weather you should be more worried about.
And yes, a metro guided tour is certainly a possibility 
I forget what festivals and stuff are going on in August but basically summer in Montreal is one long string of festivals: jazz fest, comedy fest, film fest, grand prix, gay pride, carifiesta, etc., etc. There is pretty sure to be something going on here when you come.
If you like walking, we have some real beauty spots: I strongly recommend walking up the mountain (a very easy stroll from the bottom, but if you prefer not to, you can also take the bus to the top) – there are wonderful views of the east end and downtown, and the forested park is attractive in its own right. As for offbeat stuff, there’s Mount Royal (Protestant/non-denominational) and Notre-Dame-des-Neiges (Catholic) cemeteries on the mountain (side by side), where several quite famous people are resting.
Other great places to walk around include the Lachine Canal exercise trails and the Parc Jean-Drapeau on two islands in the Saint Lawrence (excellent views of downtown).
Everyone gravitates to Old Montreal, which is also interesting and attractive to walk through, but get a guidebook for a bit of context. I give a half-decent walking tour of the modern downtown, as well, with various odd little tidbits that a lot of people don’t know about.
You mentioned not liking art museums. Do you like other kinds of museums? Because we have some very interesting museums on the history of Quebec and Canada (the McCord Museum of Canadian History, the Marguerite-Bourgeoys museum about a nun who was the first teacher in Montreal, the Pointe-à-Callière archeological museum, etc.)
Out east, there’s the Olympic Park. Go and gawk at our $1.4-billion monstrosity of a stadium. The Biodôme is a fabulous semi-zoo (no cages – you walk through the habitat); I don’t know whether the little one is of an age to be interested in animals, but plenty of adults go there as well (not just with the kids). There’s also the beautiful exhibits at the Botanical Gardens nearby, and you can go up the stadium’s tower.
You can’t go to Montreal without seeing churches. The best one is Notre-Dame Basilica in Old Montreal (its interior is jaw-dropping); if you can make time to see Saint Joseph’s Oratory (biggest church in Canada) and Marie-Reine-du-Monde Cathedral, so much the better.
For “off-beat” in this department, check out Christ Church Cathedral, which was once suspended in mid-air as a shopping mall was built underneath it, and Nostra Signora della Difesa, with its mural of Mussolini. (Saint-Leon-de-Westmount apparently has another mural, by the same artist, featuring Attila the Hun.)
That’s all I can think of at present. I bet the Doperealers would love to meet you as well!
… have fun!