I've been thinking about this lately. How hard would it be for Tim Hortons to implement a deposit system for re-usable plastic cups? A deposit cup could cost two dollars, and every time you buy coffee you hand in your dirty cup and they give you a clean one. To encourage people to use the deposit cups, charge 10 cents each for disposable cups. If you want your two dollars back, just return the cup.
It wouldn't slow down drive throughs at all, because unlike using your own re-usable cups, deposit cups can be filled before you get to the window. They would need to develop some standard language to make ordering simple, but people would get used to it fast enough, especially if they make deposit cups the default after a while.
Tim Hortons is on every street corner, so it already has a strong network effect, making something like this feasible (I bet most Tims customers are regulars).
The cups would be standard, so an automated cup washer wouldn't be too difficult. They could also be semi-insulated, which would be a huge improvement over those flimsy cardboard sleeves or double cups (which is usually what happens).