you should check out the olympic stadium for sure and it'll be right next to the botanical gardens. they have the chinese lanterns annual exhibit set up. it's really cool, i went last year, and will probably go again this year. lot's of photos to take there! and it's at night.
you won't need to rent a car if you're downtown. you can walk anywhere(except the olympic stadium) in about 15 minutes. there's the gay village if you're not too phobic which is usually full of cool buildings and often decorated. i'm not sure, but it's possible the streets are still shut off. that is at beaudry metro. you can walk the whole ways ste catherine street.
you should check out mount royal during the day and at night. there's a look out, super touristy i know, but it's gnarly because you can see the whole city lit up. during the day you can hike around and there's a massive cemetery on one side. there's also the st josephs oratory(but that's the other side) near that which might be worth it. if you're still in town on a sunday, that's when i suggest hitting up the mountain because i think people might still go for the tamtams (parc ave.) which is a bunch of french hippies smoking pot and playing on drums. but there's also these ridiculous people who dress up in medival garb and play sword fight.
we also have a lovely fine arts museum, a soso in my opinion modern art building, and a natural history museum at mcgill, as well as canadian history museum(meh?) i guess while you're at it, you'll probably check out mcgill university.
the old port is sick. the buildings are like 200 years old, absolutely beautiful, and there's often large boats in the harbor, and some old mills you can photograph. there's the lachine canal that's fun to walk around as well.
laronde of course is still opened on weekends if you want to go to an amusement park, and there's also the casino which looks cool at night, but we have the lowest winning percentages so generally not worth gambling. haha
the shoppings cool, and there's a funny ski shop in villeray called la poubelle de ski, if you need equipment. they're kind of like a warehouse of last years stuff.
and make sure you try a poutine before leaving!