Cheat a little bit. From Reds Meadow Store (7755'), take the Rainbow Falls Trail and then turn left a mile before the falls, onto the Fish Creek alternate route. You thus avoid the pumice slog up the JMT/PCT to Virginia Lake and Tully Hole. Descend into the Fish Creek canyon to a great camping area just across the bridge over Fish Creek (at 6360'). Turn left and climb gently to Ida Bell Hot Springs (at 7150'). total 11 miles.
Be advised, these popular hot springs are very definitely "clothing optional". All the clothing we saw was hanging on trees or on the ground. Be prepared to wait your turn for a pool.
There are no signs to these pools, but there are use trails. There are four pools - two down low in the meadow and two more up the hill. The higher ones are hotter. There are several large, well-used hard-packed campsites by the lower pools. If you come around a long, sweeping curve and out of the trees and are facing switchbacks, you've gone 100 yards too far. Turn around and look for a use trail going off to the left. Plan to camp here.
The next day (start early), you pay for all this with an 8 mile 2000' climb back up to rejoin the JMT/PCT (at 9190')south of Tully Hole.
Both routes - JMT and Fish Creek - are almost exactly the same number of miles and close to the same elevation gain/loss.
If you are re-supplying at Muir Trail Ranch, you take the 3/4 mile long short-cut trail steeply down 900' to the Florence Lake Trail that leads to MTR. There are hot springs pools south of the ranch, along and across the San Joaquin River. I haven't visited them, but you must ford the river to reach them. The Florence Lake Trail then leads you 1.5 miles gently south, back to the JMT near the San Joaquin River crossing.