People in Montreal (and Quebec City) seem to be crazy about bicycling, there are designated bike lanes on many of the streets and wonderful bike paths. You'll see people biking in the parks as well and plenty of places to lock up your bike.
A couple of places where bikes can be rented are CaRoule-Montreal on Wheels, 27 rue de la Commune est near the Vieux-Port or Velo Montreal, 3880 rue Rachel est. CaRoule also rents roller blades.
According to the guidebooks there are 200+ miles of bike trails, if you rent a bicycle they can probably give you a list of some of the better places to ride or you can find some maps on the their websites.
Those looking for a bike ride away from Montreal's busy streets need look no further than the St. Lawrence Seaway Bike Path.
Located on a strip of land along the South Shore, the bike path is attractive because of its great width (compared to other paths in the area) and relaxing setting (there are rows of trees on each side).
The best way to reach it from Montreal is to take the "Estacade" from Ile-des-Soeurs (Nuns' Island). Set just west of Champlain Bridge, it is a large structure reserved for bikers, skaters and pedestrians. The St. Lawrence Seaway Bike Path is at the end of the Estacade. If you turn left, you will go to the busier Notre-Dame and Jean-Drapeau islands. I recommend turning right: you will head towards the town of Sainte-Catherine and its Recreo-Parc (a recreational park with a small beach, exterior volleyball court and picnic tables that doubles as a refuge for migrating birds).
Equipment: You may want to bring a map of Montreal and its surroundings, just in case. A picnic to eat at the Récréo-Parc is also highly recommended!
Rent a bike, go down to old Montreal and follow the bike paths along the Lachine Canal. There are bike paths that cross the entire island. If you have a lot of energy, bike up the mountain. OK, it's not exactly a mountain, just a big park in the middle of the city. That's where we got our name. Get it " Mont Royal". Oh yeah, you can rent rollerblades in the old port and have a great time down there as well. The old port/Vieux Montréal is actually the largest tourist attraction. I find it is even more beautiful in the winter. Of course, you won't be rolling very far in the snow :-)
Equipment: Bicycles can be rented in various locations (one good place is on the corner of avenue des Pins and de Bullion Street) - ask around. Rollerblades and I bicycles can be rented in the old Port around the piers.
If you want to ride a bike or rollerblade, you can head out to the islands. Get off at Ile Ste Helene and head across the bridge in the direction of the Biosphere. When you get to the other side of the bridge, you'll see this giant track that is perfect for doing some leisurely laps.