We took the Johnston Canyon Icewalk (White Mountain Adventures, 403-678-4099) and were very impressed with our guide.
She knew many interesting things about the canyon's history, wildlife and habitat, and she was a delightful conversationalist. At the end of the canyon, you might be lucky enough to watch ice climbers work their way up the frozen waterfall. You should be in good physical shape, since you are hiking along the canyon on paths that go up, down and around natural formations.
I hope you are renting a car, simply because there are some beautiful drives in the area and it allows you to go out looking for wildlife. We've seen elk, mule deer and bighorn sheep. Also a lone coyote. Don't forget your camera.
There are also some great museums if you are into that: Banff Park Museum for the story of early tourism and wildlife management, and a taxidermy collection of animals indigenous to the Park; Cave & Basin National Historic Site, where the hot springs were first discovered; Whyte Museum of the Canadian Rockies, for historic and contemporary art and historic homes; and the Buffalo Nations Luxton Museum, for Plains Indians history.
Shopping is an obvious, especially for local art and food items. Hope you're bringing an extra suitcase!