SMS/MMS to and from Canada is included in just about all modern plans for the last 5 or so years at least.
This includes SMS while in Canada, to/from the USA/Canada too.
MMS is included to Canada also, but it will cost data while in Canada, so just disable data roaming to stop any incoming fees.
SMS to other countries costs a rip off fee of 20 cents to send/receive while in the US.
Install and use google voice, this will allow free SMS to Canada from anywhere, using data (and wifi), while in the US, configure it to use GV when dialing internationally, it will give you no cost calls to Canada.