The String Lake area of the Grand Teton National Park is very popular, largely because the calm waters are a perfect setting for canoeing, kayaking, or simply taking a plunge on the lake. Several hikes are accessible from the picnic area by the lake as well, including Hidden Falls and Leigh Lake Trail, among others. One of those hikes is the String Lake loop, which is fairly gentle and offers great views of the lake.
The parking lot has gotten increasingly busy over the last few years, but there is usually enough space for everyone. There are picnic tables by the shore, and restrooms at the north parking lot. Assuming you would like to make the hike clockwise, start walking south toward Jenny Lake and cross the bridge over the creek at the south end of String Lake. Do not miss the view of Grand Teton from bridge. Turn right 1,000 feet later to continue around the lake (head left to hike to Jenny Lake and Hidden Falls).
From that point, the trail ascends 150 feet over the following mile, on the west side of the lake. Look for huckleberries and other wild berries in the summer. The next half a mile is fairly flat before the final ascent of another 150 feet or so. After that the trail loops around and descends through the forest for a very pleasant walk. After crossing the bridge, the trail overlaps with the Leigh Lake Trail. Take a moment to walk north to Leigh Lake, where Boulder Island makes for a great foreground to Mount Moran in the background. The walk back (south) to the trailhead is mostly flat, as Grand Teton is reflected on the surface of the lake if it is not too windy.
Grand Teton National Park is grizzly bear country, so it is recommended to hike in groups, make noises -possibly carrying bells hanging from your backpack, and carry bear spray. As the area is usually fairly busy, it is unlikely that you will need it, but it is always a good idea to have it just in case. Other wildlife that can be spotted in the area includes coyotes, deer, elk, moose, black bear, marmot, eagles and osprey.