I grew up in Calgary -- and the bad news is that March/April is still the tail end of winter... if you're a skiier its the best time of year to be up there -- still great snow, but relatively warm (between -5 and +10 celcius) and the Banff springs (or better yet the Chateau Lake Louise) are about as close to a castle as you can come in North America. The downside is that you'll have no guarantees with the weather. There could be not a cloud in the sky (which happens a lot in the spring), or it could be pretty socked in.

Banff is good for a weekend - spend a day putzing around town and shopping, and a day in the spa. Pretty good food, and cheap prices of you are coming from the US. The town itself is about 1 1/4 hours from the Calgary airport on an easy highway drive.

Whatever you do, good luck !! I chickened out 3 times before finally getting my cojones up to propose in New York (and after my best laid plans of proposing on the skating rink of Central Park it rained and I ended up getting to one knee in our hotel !!)