While definitely one of the most popular hikes in the park, Emerald Lake is also one of the most beautiful (and one of my personal favorites). Featuring three different lakes (four if you explore around Bear Lake first), spectacular views of Long's Peak, and easy terrain, it's not hard to see why so many people take this trail.
The hike begins at the Bear Lake Trailhead, a junction for quite a few different hikes. This means that during the peak months (every season except winter), you'll probably have to leave your car at the park 'n ride and take the free shuttle up. The shuttle stops at three different trailheads, but Bear Lake is the final stop.
The trailhead has a Ranger Station and a set of bathrooms, so you should be set before beginning your hike. If you're there in spring (up until mid- to late-June) or fall, it would be a good idea to talk to a Ranger about the trail conditions higher up.