First of all, Alaska currently has NO orders for the 737-800! They have placed orderes for, and recieved 737-700s and ordered the 737-900.
For now the 737-700s and 737-900s will probably be used to replace MD80s and maybe the older 737-400s, but I think those will be here for a long time!
As far as I know, Alaska DOES NOT plan on going to any more Canadian destinations. They currently only serve Vancouver. Horizon Air, Alaska's commuter partner flys to Victoria, Vancouver, Kelwona, Edmonton, and Calgary. The 737-900s will probably be placed on runs up and down the west coast or from Seattle to Anchorage and Fairbanks.