We did this trip two years ago. At Banff, you can walk into town for some shopping or visit the hot springs. At Jasper, you can rent kayaks or bicycles (which is what we did), or just relax on lakeside chairs and enjoy the scenery. Your Tauck director will have lots of suggestions. We loved this trip. You'll have a great time.
It's been quite awhile since I did the Grand Canadian Rockies, but my friend and I each did our own thing when we were there. We didn't do anything in Lake Louise because it's not near anything (or it didn't used to be) so we just hung around the hotel and walked by the Lake. In Banff, we walked into town and they also offered horseback riding. On our tour, we were only there a day, so we just went into town. At Jasper Park, I took a wildlife tour in a small van and went horseback riding. My friend took a tram to the top of a nearby mountain and also rented a bike. We also rented a paddleboat and paddled on the lake at the hotel. None of these was included in the tour. This seemed to have the most choices of things to do in our down time.