As you appear to be aware, most of the attractions are in Old Montréal and downtown along St Dennis, St Laurent, St Catherine, and Sherbrook streets. There are two Marriott hotels in Montréal that are very convenient. The Springhill Suites in Old Montréal is close to the old port of Montréal and the Notre Dame cathedral. The Courtyard on Sherbrook is a stone-throw away from St Laurent and St Dennis streets. Both are close to St Catherine, and both are close to subway stations. Things to do... depends on what you like to do. Shopping in Montréal is amazing. Along St Dennis and Mt Royal, there is an endless selection of independent boutiques. If you like food, you can find food from every country along St Laurent. Since you're in Québec, don't miss local delicacies like poutine. On St Laurent, there's a deli called Schwart's. Their smoked meat is out of this world. For strolls and parks, take the #10 bus from the Mt Royal metro stop to the top of Mt Royal. There's a park with a lake there that's good for a romantic stroll. If you feel up for it, you can rent a bicycle at the old port and bike the river-front bike trail. Make sure you also visit the underground city (RES) along St Catherine. It's a large network of interconnected underground malls and eateries that lies beneath the street. There are a few other Marriott hotels in Montréal. The Marriott hotel is a bit far from all the action and so is the Residence Inn in Westmont. I hope this helps.
We had a great stay at the SHS Old Montréal. There's plenty within walking distance and it is a nice area. One thing I should mention is that it is basically situated on an alley. I forget how they play it up...something like "nestled on a quaint side street", but basically it's an alley. It's not dangerous or anything like that, I just felt it was somewhat misleading. Walk half a block in any direction and you're in the Old Montreal you came to see.