Back in March, I posted asking about visiting the Canadian Rockies with my teen daughters. https://community.ricksteves.com/travel-forum/beyond-europe/calgary-and-the-rockies
I wanted to let you know how the suggestions worked out. After they looked at pictures of both places and we talked about the relative crowds at Banff/Jasper vs Waterton Lakes, they were in favor of visiting Waterton Lakes National Park. Since we stayed on weeknights, I was able to find hotel options, but starting earlier would have been better.
We flew into Calgary late on Saturday and stayed the night to sleep before heading to Waterton for two nights where we hiked in a canyon, hiked up to the Bear's Hump overlook, went canoeing on Cameron Lake, took the lake cruise, and ate at the Prince of Wales hotel. Then back to Calgary for two nights with a day trip to Kananaskis for rafting and a day with relatives, and then a night in Drumheller with the Royal Tyrell Museum (dinosaurs).
Waterton was a good decision for us. Beautiful, not too crowded on weekdays so we had serene moments, a variety of activities. Our two night stay with two half days and a full day was somewhere between the minimum for the highlights and about right - we could have filled another day or two with hiking or horseback riding, but I don't regret choosing rafting and Drumheller instead. We were moving around and had a couple of hours driving most days, but the scenery and roads were good.
One warning is that Waterton had a massive wildfire in 2017. The undergrowth was green, and purple flowers were everywhere, but the charred skeletons of trees are also still standing. New trees are growing, but it is slow. Still beautiful, but in a harsher way than areas that fire hasn't been through.