Johnston Canyon (take it all the way to the Ink Pots) is the most amazing hike we did in Banff. The tea houses are nice, but those trails are crowded. Most people don't hike beyond the falls in the canyon, so the crowds thin out significantly. My girls (7 and 11) did this and loved it. You climb up into a high meadow with the river and ink pots, and you're surrounded by these rocky peaks. It is beautiful!
The Vermillion Lakes are a fun side trip, too. They are easy to explore and not too crowded.