Emerald Lake is as beautiful (or even more so!) as the name implies. This gorgeous lake is surrounded by snow capped mountains and even a couple of glaciers. I find that the hue of the lake varies on the time of day and weather.
Enjoy the views, rent a canoe ($100/hour) or walk the family-friendly 5.2km loop trail around the lakeshore. Allow around 2 hours for the latter. Emerald Lake is the cheapest of the 'big lakes' in this area for renting a canoe and the only one that has a trail all the way around it.
Emerald Lake is less busy than Lake Louise but I would still plan to arrive here early if you can (before 10am). It gets quieter again in the afternoon, after 4pm or so. '
A Parks Canada parks pass is required to visit Emerald Lake ($22/day for a vehicle of up to 7 people).