The Beaches in Cancun are horrible and the OP specifies a North American destination.
Geographically, Mexico is absolutely part of Mexico, no matter where you happen to think it is. Also, Starwood has the Mexican properties as part of North America on their site. I was in Cancun earlier this month and thought the newly reconstructed beach was pretty good, but different strokes for different folks, I suppose.
And since some airline programs do charge more points for Mexico, I'll add a few US suggestions.
From Western Canada, California might be a good choice, as the flights are easier with a little one. Choices here include the Sheraton Carlsbad or Sheraton La Jolla, Westin Mission Hills, and a variety of San Diego Properties.
If you're in Eastern Canada, Florida might be better, and choices there include the Sheraton Sand Key Resort or Westin Coral Gables.
Hope that helps,
Michael