ALW, you beat me to the post. It is illegal for a Canadian Resident to drive a US plated vehicle at any time in Canada for leisure purposes. This also extends to visiting family and friends who might wish to loan you their car while visiting Canada or want you to chauffeur them around. The prohibition is noted in article 4 as follows:

Under no circumstances are conveyances admissible under these Regulations for touring purposes or for other leisure activities in Canada, nor is any local use permitted (e.g., point to point movement in Canada).

mxm135