If you are driving along the Icefields Parkway, a stop at Peyto Lake is an absolute must. Yes, it's only a short trail up to the viewpoint and it can be very crowded up there (so be aware of that)...especially in the summer months. If you can visit either before 10.00am or after 4.00pm and you may be able to avoid the big crowds. And if you have some time and want to get away from the crowds you should follow the Bow Summit Lookout Trail which starts at the Peyto Lake viewing platform.
Many people only walk the paved nature trail to the Peyto Lake viewing platform. However, if you continue past the viewing platform and climb another 3km (1.9 miles) to the Bow Summit Lookout you will find some amazing views away from all the crowds. In my mind definitely worth it if you have a couple of hours for this hike. Here is a link to a PDF file with a very good description of this trail: http://www.summerthought.com/50Walks_sample.pdf