Happy to hear you are on your way out to CA for some fun.
I recently purchased a Hennessey Hammock, and through lightweight camp website links, found and bookmarked your blog. I have received many ounces of knowledge thanks to you!
I was happy to be able to "repay" the favor, but it looks as if I will be re-hashing earlier posts that have explained the two parks well enough.
Before I read them, I had thought Yosemite was much too far away for you to get a 2.5 day camp at. The drive is listed at 5 hours, but this does not include the LAX traffic (although you will be getting the northern county only, and will be well past it if you take Interstate 5 through the "Grapevine") and of course, the slower speeds once you near Yosemite. And though once you drive into Yosemite, and can see El Capitan, Half-Dome, and the other gorgeous rises along the glacial-cut valley, I am not sure you will be happy to realize you'll soon need to turn around and drive back out in one day.
I was therefore going to suggest Joshua Tree. I just went there two weeks ago for a three-day trip, with the intent of trying out my new hammock. The park is gorgeous with the seasonal Ocotillo in bloom (magnificent).
Though some have mentioned that the park would be empty,the ranger signs had posted FULL at every site. We were able to find a few open spots toward the less popular southern area, but it was way crowded.
From reading your blog, I had thought you might like to enjoy a solo-trek, off trail, into the desert for some enjoyable camping at the southern edge. I read that you are concerned about heat, and it seems that the weather report shows cloudy conditions. I love this for the desert, but that usually means high humidity.
My only hesitation in following through with suggesting a trip to Joshua Tree is that you have already been there, and you have not yet seen Yosemite. I would love then for you to see it, but must ultimately say that you visit Yosemite when you have more time. An over-nighter with two days of driving doesn't sound good, when your other option, Joshua Tree is at one of its most beautiful times of year.
Best of luck, and I'll try to check back in case more advice is needed, or I can help others. I'm trying to head out for some southern California surf/camping this three-day weekend, but have not yet set the locale.
Here's to all of us finding a nice spot to rest our feet this weekend.