I highly recommend the June Lake Loop, it was one of my favorites when I lived up there. Take the back road out of Mammoth Lakes, it cuts off from the road up to Mammoth Mountain (can't remember the name right now). Head north on 395, then cut off on the June Lake Loop. About 55 miles out and back. 395 is nice and wide.

http://www.gmap-pedometer.com/?r=2013699

It's also very scenic (and demanding) to ride from the town of Mammoth, up to Mammoth Mountain resort, then down to Devils Postpile/Reds Meadow, then back up and over Mammoth Mountain to the city. Be sure to stop and do the 1/2 mile hike to Devils Postpile, it's really cool if you've never seen it before. The ride is probably less than 25 miles, but lots of elevation. The road down can be a little tight, but it's closed to public traffic.