One of my favourite bike routes crosses the Jacques Cartier Bridge. Its bike path begins close to the Papineau/La Fontaine intersection and connects the island of Montreal with the Saint Helen’s Island and the south shore. You don’t want to bike all the way to Longueuil, but leave the bridge at Saint Helen’s Island. The bridge has two small platforms that give you the possibility to take in the view without being overrun by cyclists. If you don’t feel like biking up and down the bridge twice, you can either come back by metro or turn around on top of the bridge.
I recommend to cross the bridge by bike, although it is possible to walk up the bridge. Be aware of cyclists as they might be coming down fast. You might not want to do this during rush hours, as there will be a lot of commuters using the bike path.
In summer, the bridge is often closed off to traffic on weekend nights because of international firework competitions.