Here are your best options (in my opinion):
If you want to DH. Head to bluemountain (bluemountain.ca). In Collingwood. Bike rental on site (DH and XC). You are looking at about a 2 hour drive. The area is nice, you may just want to go for the weekend.
There is also exceptional XC riding in and around the area. 3-stage and Kolapore by name. I have not ridden them, but hear they can be tough to navigate without GPS or a guide. If you have a GPS, check out the post recently with all the data.
If you want buff, fast XC trail riding, your best bet is Hardwoodhills (hardwoodhills.ca). Bike rental on site and an extended trail systems for an extended ride. ~1.5 hours North of the city.
Albion Hills may get mentioned to you. Another favorite spot, however it will be hosting the largest 24 hour race that weekend in North America. Stay away, unless you want to witness the sight.
There is lots of other riding in the area, but the two mtb centers mentioned above are probably the easiest for a visitor to access MTB.