I'd recommend choosing one of Lake Louise or Moraine Lake. LL is nice, and the trail around the lake is nice, plus the Lake Agnes teahouse is there, but the trails, especially towards the weekend, will be swamped with people. Moraine Lake is nice, and the trails seem to have less people, though not by much.

Either way, arrive BEFORE 10 am or you're going to struggle to find parking.

Johnson Canyon is a nice easy hike for someone who's looking for a good payoff without too much effort. Note, though, that if you get there after about 10, you'll be in 'rush hour' traffic in the parking lot and trail.

If you're driving to LL/Moraine Lake/Jasper from Banff, an option is to drive the Bow Valley Parkway instead of the Trans-Canada. Slower drive, no wildlife fences, great views (TCH has some good views too) but a much better chance of meeting animals on or on the side of the road.