Hmm... loop cuts a number of good options out. While not quite a loop, I would suggest South Lake to North Lake in on the eastern side of the sierras. It's not a classic "loop" going in and out the same trail head, but the trail heads weren't that far apart... something like 5 miles apart. When I did it some nice day hikers gave me a lift so I didn't end up hoofing it between the trailheads. This hike goes from South Lake through Bishop pass, down to JMT, Over Muir Pass, through Evolution, Over P? Pass, and out to South Lake. There are a number of variations, especially if you do a bit of cross country. Alas, don't have good picture, camera died the last time I did this trip.