I have spent the night in the parking lot at Mosquito Flat. Since it is at 10,000', it is a good acclimation site.
Another hike out of the same trail head, but involves 2k elevation climb, is to head over Mono Pass into Pioneer Basin. It is very pretty up there.
If you head out of Onion Valley, maybe spend the night at Charlotte Lk, and dayhike over Glenn Pass to Rae Lakes. That is one of the most beautiful places on the JMT, in my opinion.
There is a shortcut out of Charlotte Lk to Glenn Pass--just head north up the drainage. Near the top, it heads right and joins the JMT. (it's an abandoned trail) Over Glenn Pass and back in a day, though, might be a long day.
Lower Rae Lake and the Fin Dome
Click the picture to see the album.