If you’ve not driven in the US before, I wouldn’t recommend LA or SF as your first place to do so. Think London at rush hour with worse roads and even more aggressive drivers.
if you’re going to visit LA and want to the Hollywood/Downtown tour, I highly recommend booking one of the guided tours - I’ll try to dig out the website of the one we used if anyone’s interested.
You get driven around in a cool, clean minivan by a guide who knows the place like the back of their hand and because they have special license plates, they get to park right outside the place you want to visit, for example right outside the Chinese Theatre where the Hollywood Walk Of Fame is.
Universal Studios is flat out brilliant. For a once in a lifetime trip, splurge on the VIP tickets. You get valet parking in a very easy to find parking lot (the regular one is the size of Slough), walk straight onto all the rides, get an excellent buffet lunch and a 2 hour back lot tour where you actually get to walk around the sets and visit a prop warehouse. My top tip is for each person to have a small shoulder bag/man purse to put wallet, phone, sunglasses in as you need to remove these for the rides. Our guide was happy to hold these for the party of 12 we were in.
In LV, go to Battlefield Vegas. Want to show the real version of your favourite CoD/MoH/Battlefield guns? No problem, just bring money. If you’re staying on The Strip, they’ll actually come pick up in a real Humvee. Take lots of money as you will want to shoot more than the package you’ve decided on. I paid for the wife and daughters to all shoot a Sig 9mm handgun and I got to put a belt through a Soviet light machine gun. There’s a minigun if you really want to empty your wallet in 5 seconds or less.