Just west of Field, BC in Yoho National Park, you'll see the turnoff from Highway 1 to Emerald Lake. The drive takes about 15 minutes from the highway, with many other sites along the way.
Emerald lake deservedly gets its name from the emerald-like colour of it's waters. Glacial water flows into Emerald lake giving it that beautiful emerald shade.
There's a 5.2 km light-walking trail that circles the lake. There are also many interperative signs that tell you about the lake, the surrounding mountains, and the eco-system.
The last time I was at Emerald Lake, there was actually a rockslide far up in the mountains above the lake. There was the sound of thunder and rock hitting rock, and we saw a large dust cloud rising off the mountain. While there was no danger to anyone around the lake, it shows that the Rockies are a very active and wild place and you should exercise caution.
When you pull into the parking lot for Emerald Lake, just because you see a number of tourists, don't think for a moment that wild aimals such as bears won't be in the immediate area. Exercise caution. I've seen a black bear on the trail before, so use the usual precautions to avoid any unwelcome encounters.
You can rent canoes and go for a paddle in the lake, as well there are some cabins for rent and a small cafe/restaurant where you can fill up.