If you are going during prime season, then Guitar Lake can be very crowded since it is the most strategic spot to camp for an early morning stroll up to the summit. There are some other ponds and streams in the area, but this year's drought might be doing ugly things. The ranger from Crabtree used to go make the rounds at Guitar Lake every evening, mostly checking on permits, checking for WAG bags, and harassing people who camped too close to the water.
Some people are able to go from Guitar Lake to the summit, then all the way out to Whitney Portal by Miller Time. I suppose that you can do that if your knees are in really good condition. With some fear there, some folks leave Guitar Lake super early so that they can be on the summit for sunrise. That's admirable.
Carrying a partially empty backpack up to the trail junction might be pretty tiring. Then, once you return to it from the summit, you have to ascend a few hundred more feet to get over Trail Crest. I hate that part each year.
Going down the Switchbacks, it is mostly graded OK. Once you get below Trail Camp, there are lots of steps that take a toll on your quadriceps, but that lasts only for a couple of miles.
–B.G.–