The Gabrielino National Recreation Trail might be what you're looking for. It starts at Chantry Flat and ends at JPL. Plenty of water along the route, and several trail camps to choose from. It can be done in two days, or a very relaxed three days.
Check the weather, though, it can be hot this time of year.
The San Gorgonio high country is really good this time of year. You could start at South Fork TH and camp at Dry Lake, or Dollar Lake and hike several of the peaks in the area.