Unless you can get a good deal on Canadian roaming through your US provider, I think that your best bet would be to unlock your phone and pop a Canadian sim in there. While looking for a plan for my Dad, I came across this site:
http://www.comparecellular.com/
That will give you a good idea as to what's out there. You'll want a "prepaid" plan which is basically a "pay as you go" sort of thing. The other plans are contracts.
A few caveats though: There are really only 3 big providers in Canada: Rogers, Bell, and Telus. They each have a discount brand : Fido, Solo/Virgin, and Koodo respectively. There are a number of other brands out there, but they do often come with coverage limitations. Wind, for example, which has already been suggested, might give you adequate coverage in major urban areas.
I can't speak personally about how good or bad Wind, etc. are as I have been a Fido customer for quite some time. Perhaps someone can chime in with their own experiences. The one major caveat is that we are pretty gouged for wireless in Canada, and although the US is also pretty horrible for prices and options, just be prepared to pay a good chunk for what seems like a small amount of minutes and megabytes. And customer service? Forget about it.
