Grant Lake CA State Route 158
There's a derelict day use area at the South end of Grant Lake, just down the road from Lee Vining and Mono Lake. (At least it looked derelict, the restroom doors had been hacked down and garbage had accumulated there, but the rest of the area was relatively clean.)
We visited late in the season, and the campground we planned on using was closed. While looking for a turnaround spot we parked here, it was late, and so it ended up being an excellent spot to spend the night.
I would expect it to be much busier and maybe patrolled during the season, but with the CA state budget what it is...
The Sierras really are nice!