Mont-Tremblant is a tourist town, famous for its ski hills. The rates are lower in summer, but you might still have difficulties getting a good rate via Priceline. It's connected to Montreal by a highway (15 or 117), so you can get there about 2 hours. The highway passes through a region known as "The Laurentians"... also a good tourist area. Its highlights are: ski resorts, romantic French restaurants, waterpark, many lakes for fishing or boating, etc. If you are short in time, you should stop at The Laurentians (1-1.5 hours drive) as Mont-Tremblant is not that interesting and that different in the summer.

Instead of driving, why not take train between Montreal and Quebec? Via Rail is very comfortable and affordable. You don't have to worry about getting lost. In fact, if you are into trains, you can get a rail pass for about US$140 and get to see Ottawa, Montreal, and Qc City. Once you are in Quebec city, just take a cab or pick up a rental car there.

http://www.viarail.ca/planner/en_cart_corr.html

Car rental in Ottawa can be as low as US$10 a day, unlimited mileage... I have a coupon for it so if you are interested, let me look for it.