There's 2 excellent web-cams at the top of the Banff Gondola. You can check them out online at
http://www.explorerockies.com/banff-webcam/
- or they have them live on big-screens at the lower terminal, so you can check the weather up top before buying tickets.
The Banff Park Museum at the end of Banff Ave before the bridge is good, or the Whyte Museum is excellent - it's across from the town hall and often has children programs on - or kiddie 'museum packs' with activities. If you can join a Heritage Home tour with them, it's worth the time.
http://www.whyte.org/programs/family_programs.html
A cool trip is going to Simpson's Num-Ti-Ja Lodge, which is 40 km past Lake Louise on the Icefields Parkway. It's a real interesting building with ancient taxidermy in the dining room - ask if you can peek into their guest lounge too.
Might want to phone ahead to see if it'll be open for lunch
http://www.num-ti-jah.com/
Good choice on the Castle Mtn cabins. If you're looking for a little change next time, try Baker Creek Chalets just a little further up the Bow Valley Parkway.