>>2867604you goofed, that time of year is great for Yosemite, the waterfalls are incredible, though there's no snow this year so they'll probably dry out soon, and awful for Death Valley and Joshua Tree. You should really save those for a winter trip, you can do Grand Canyon, Zion and Bryce too, they're all great in winter. Some nice areas near Vegas also.
Channel Islands you should camp overnight if you can, probably too late to book though. The kayak tours are expensive but I thought it was worth it. Snorkeling was nice too, but frigid.
In Sequoia the Giant Forest area is great but gets busy, start as early as possible, and go beyond the paved paths, there's a pretty extensive trail network to design your own loops out of. I also recommend Muir Grove, it's not the best grove but it's pretty good and because the campground is closed it will probably be empty.
Even if you somehow get lucky and it's "only" in the 70s or 80s in the desert, the midday sun will still bake you alive. If it's 90s or higher, forget it, just do shorter hikes right around sunrise or sunset.