Johnston Canyon's suspended metal walkways provide secure passage through the deep gorge to a series of waterfalls. It's an interesting experience to be in the middle of the canyon, rather than at the top or bottom.
The 2.4km return hike to the Lower Falls takes less than 1 hour. The Upper Falls are 5km return. Johnston Canyon is usually very busy from 10am onwards, so I would recommend getting here early. Otherwise, visit in the evening.
In the summer months, a line can often form to view the Lower Falls through a cave (up to 45 minutes!) In my opinion, this is not worth it. The falls can be seen from the stairs that lead down to this cave. And besides, the Upper Falls are more beautiful (and no line).
If you have the energy and want to see more, head beyond the Upper Falls to the Ink Pots (additional 3.1km one way with 220m elevation gain. These aquamarine coloured pools have a lovely mountainous backdrop. The trail is usually a bit quieter than the rest of Johnston Canyon.
A Parks Canada parks pass is required to explore Banff National Park ($22/day for a vehicle of up to 7 people).