We're probably going to stay in WeHo. LA is the one place we haven't sorted out accommodation. We're trying to work out where makes the most sense. I think most of the evening stuff we want to do will be around WeHo, and it's relatively easy to get from there to Santa Monica and Venice, and I think most of the touristy stuff we will do will be in that area. If we do a theme park it will probably be Universal. The Wizarding World of Harry Potter should be open by the time we get there.
If you're in that area, I recommend the Museum of Jurassic Technology. I've been at least four times, and it's been a hit with everyone I've taken there.
If you want to have a celebrity sighting, the best place is a morning movie at the Arclight Hollywood. I've also seen a lot of people at Little Dom's, a restaurant in Los Feliz, but that's on the other side of the city.
Don't fall in to the trap of Hollywood Boulevard, and don't get your expectations up TOO high about Universal Studios.
The Warner Bros. studio tour is expensive but worth it IMO.
I don't know how you get up to Griffith Observatory on public transportation, but it's a great first thing to do when you get to the city. It helps orient the geography. LA's kind of a difficult place to wrap your head around.
Any random street taco is going to be better than almost every restaurant. People have a lot of fun tracking down the Kogi truck, which serves Korean tacos & burritos. Be prepared to wait in line and make a mess all over your shirt.
If you like comedy and are free on a Thursday night, there's a free show at The Palace Chinese restaurant in Los Feliz that always has walk-ins of the best alt comedians.