According to Foreign Affairs, you do not need an IDP to drive in Spain. Their web site is usually up-to-date and accurate. (They recommend an IDP, but that's their standard statement for all countries. When it's required, they say so.)
See item 4 in
http://www.voyage.gc.ca/dest/report-...country=274000
I have rented a car in Spain and have not been asked for an IDP. I wouldn't bother.