I've done it. The drive from SF to LA sucks and quite long. If you hit rush hour expect an extra 2-3 hour delay. You'll be still bumper to bumper for MANYYY miles. Not fun.
Six Flags was not fun when I went. Expect 1-2 hour long wait for ONE ride. And it's pretty grundgy. Was expecting a beautiful theme park. If you love thrill rides then it might be worth it to you.
BUT unless you pony up and buy the at least the gold flash pass (extra $70/person) or platinum (extra $100/person) don't waste your time.
Vegas will be hot and require ALOT of walking. You're only spending one day and probably want to check out as many hotels and stuff. That will pretty much be your whole day. You won't have any more energy after that guaranteed. If you do have energy maybe check out a show. Cirque shows are good, but quite expensive. There's booths thats sell discount last min tix (
Las Vegas Discounts | Half Price Tickets Every Day | Tix4Tonight).
Sounds like you'll be driving as much as the sight seeing.
SF is very beautiful. I drove down to visit a friend, originally planned to stay the weekend. Ended up staying almost a week. There's wayyy too much to see in SF.
I would honestly spend more time in the cities you haven't been to before. You're cramming so much in such a short time you basically don't get to see the actual good parts of any of the cities.