So first and foremost - as someone who lives in FL and lived for a while in LA as a passholder at USH, below are the attractions (I believe) you should hit up and aren't duplicates.
Universal
- Harry Potter and the Escape from Gringotts
- Diagon Alley (I put this as a stopping point because there is a lot to take in on a first visit, especially as a potter fan)
- Men in Black: Alien Attack
- E.T. Adventure (if you didn't experience it before it left USH)
- The Bourne Stuntacular
- The Universal Horror Make-Up Show (has some bits from the Special Effects Stage at USH, but mostly different and way funnier)
- Race through NY with Jimmy Fallon
- Revenge of the Mummy (it's different and IMO, way better than USH. Goes backwards less, but is a solid 60-90 sec longer)
- Rip Ride Rockit (a lot of people on the forum don't like this ride, it is rough, but the music choice element makes it a try for me if you like coasters)
Islands of Adventure
- Skull Island: Reign of Kong (it is different from USH, the Studio Tour only contains the tunnel portion, and with much worse 3-D effects...this is more of n all-encompassing 5-6 min adventure - highly recommend)
- The Amazing Adventures of Spider-Man
- The Incredible Hulk (if you can stomach intense coasters)
- Hagrid's Motorbike Adventure (This is a MUST if you like Potter or Coasters. It's probably the least intense coaster on this list, but it's arguably my favorite ride at the parks now...and I say that as someone who has been on it about 15 times)
- Popeye's Bilge Rat Barges (it's a really good rapids ride...if you're okay with being soaked, go for it)
- Dudley Do-Right's Ripsaw Falls (this one i almost left off....it's kind of a pale Splash Mountain to me. Poor effects, but a heck of a drop...and yes you get soaked on this one too)
- Cat in the Hat (maybe do this if you really loved the book as a kid...but it's in bad shape now)
So....if you tried to do all these things, it would be hard in 1 day. Maybe a day and a half, but I'd say 2 to be safe, especially if you're going on a weekend.
I went on a Monday last week and had a blast. The social distancing measures are mostly good and people generally remain safe, though I did see quite few occasions of improper mask wearing or people taking them off.
My recommendation? Go on a weekday at rope drop for Islands. I think it typically opens at 10 while Universal opens at 9. I got in and went straight to Hagrid, got on it twice in 10 min. That's truly never happened before and probably will never happen on any other visit, but you also might get lucky.
Most of all, take your time, don't stress out and be safe. I personally didn't feel comfortable doing indoor dining, so I did counter service and ate outside. Which by the way, make sure to download the Universal app in advance, upload your ticket (you can get virtual reservations day of for some rides), and you need to order food in advance via the app to pickup at locations that aren't table service.
EDIT: As for Disney, if you have the ability to go to Disneyland and DCA more frequently or have been before, I recommend visiting Animal Kingdom and EPCOT during your stay. They have the most attractions and things you can't do or see in CA.
I hope you have a nice trip!!