Depending on your budget/taste, the Banff Springs might be a good option as well. At 3-400/night, it's a bit expensive for a quick weekend, but Fairmont has a monthly sale from the 1st to 2nd wednesday of every month, and the Banff Springs has select availability for $159/night.
Only weekend I could find was the first weekend in May, $159/night for a room with 1 Queen bed. Upgrade to the King was $199/night.
More info here:
http://www.fpcnews.com/greatrates/index.aspx
Must book by tomorrow (April 12)
Edited to add: Jasper Park Lodge is also a neat place for a romantic weekend, and has more availability on the same sale. It's also only $149/night, but it's quite a bit further to drive.