Bicycle
With a bit of careful route-finding homework, Montreal can be a great place to get around by bike. It's mostly flat and a network of cycle-lanes along selected roads keep bikes out of traffic for the most part.
You can also go surprisingly far on good dedicated cycle-paths, the best of which go from the Old Port south and along the Lachine Canal and over to Parc Jean Drapeau where you can even ride on the Grand Prix track when it's not in use.
Don't forget the possibility of using the Navette Maritime shuttle to cross the river and ride back - which will add a little mini-cruise to your bike ride.
Rentals
A selection of different types of bike can be hired from Montreal on Wheels on the waterfront in the Old Port from $9/hr or $35/day.
Another option are the sturdy BIXI bikes of Montreal's bikeshare system, though their pricing structure is designed for short hops of 30mins or less, which isn't conducive to relaxed city exploration.