Sydfly,
Way too busy for a week--you'll spend most of your holiday in that bloody car & have very little time to enjoy anything! It's the same as trying to drive Sydney--Blue Mountains--Canberra--Melbourne--Newcastle--Sydney in a week!

Canada has the excellent VIA Railways which go to most of your destinations (except Mt. Tremblant & maybe Algonquin). I would recommend flying into YYZ & out of Québec City (YQB) or v.v. & taking at least 2 weeks, as U need at least 2--3 nights in each city to get anything out of it.

Mon. arrive YQB;
Wed. or Thu. train (3hrs.) to Montréal;
Sat. pick up rental car, drive to Mont-Tremblant or Algonquin, then the other on Mon.;
Wed. drive to Ottawa, return rental car (maybe--depends on weekly rate).
Fri. train or drive to Toronto or Niagara;
Sun. bus, train or drive to Niagara Falls/ Toronto;
Mon. or Tue. back to YYZ to fly onward or home.

Please note that weekends are cheapest in Toronto hotels but dearest in Niagara Falls hotels. Also, Ottawa is best done during the workweek because some of its main attractions are closed Sat.--Sun. & it's less of a weekend party city than the others.

Also, Hertz considers YYZ, downtown Toronto, & Niagara Falls all one city for pricing, so it should be cheap to pick up in one & return to another of these. Also, pickups at YYZ are subject to a 15% Premium Location Fee similar to an Airport Concession Fee of the USA, while in-city pickups are not.