[CaptainCrunch]

http://ezinearticles.com/?Taking-a-R...der&id=1124788

You have a lot more leeway here than if you are wanting to cross the border into Mexico. The majority of car rentals in both Canada and the U.S. will allow you to take your rental car across the border, as long as you are planning to come back again. The exception to this rule is if you are a resident of the country that you are driving into. For example, if you are a Canadian citizen, you cannot take an American rental car into Canada unless it has Canadian license plates.

Also, if you are going to be taking a rental car across the border into Canada, you will need to show your rental documents to the border officials, so have them handy. In addition, you will probably need to purchase additional insurance, since the insurance that comes with the car rental won´t cover accidents or theft outside of the U.S. You will probably need to have a Canadian non-resident insurance card in this case.
Talk to your rental company before attempting to cross the border, since you don´t want to be crossing without permission or insurance!