Re: Half Dome and El Capitan trip
I strongly suggest Tamarack CG off highway 120. It should be fairly easy to get a site especially if you arrive Sunday through Thursday when the camps are less occupied. Tamarack is only 30 minutes from the Valley but will require driving a few thousand feet down into Yosemite valley as well as having to climb back up towards the camp so fuel is used up to get there and back. But there is 24 hour fuel + a little store just minutes from the camp at Crane Flat. There is no potable water at Tamarack but it does have vaulted heads. The nice thing about this campground is it's more centrally located. Most of the valley sites are well within an hours drive except the high country which is not too much further off.
IMO most people that only go to see the valley are missing much of the beauty the high country offers. Although spring time is the best time to visit Yosemite IMO, Tenaya Lake and Tuolumne Meadows are well worth the drive in autumn when the waterfalls are not so active. Fantastic spots along the lake to have lunch at. Olmsted Point supplies a unique view to the valley as does Glacier Point located in the opposite direction from Tamarack. Also located a few minutes from Tamarack are the big trees of the Tuolumne Grove. You can walk or shuttle to the trees. I think they should be able to fit in all of these routes unless the plan is to hike to several locations. You can spend as much time in each of the areas as you want but easily plan for a full day in the valley and the same with the high country. Glacier and the big trees usually take about 2 hours just walking around and taking it in not counting the drive. The key to Yosemite is leaving out of camp early.
Tamarack usually closes at the end of September.
All the times can vary depending on traffic and how long you spend checking out the vistas.
September is a great time to visit and I hope they enjoy their trip.
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