Jewel Lake is one of the best introductions to hiking the Rocky Mountain National Park lakes. This is another hike that begins at the Glacier Gorge trail system, so it can be crowded at the beginning.
However, once you get past the Alberta Falls, you will see significantly fewer people. It is 5.9 miles round trip and has some pretty amazing places to set up a hammock or eat lunch while enjoying some of the best scenery the park has to offer.