Probably the highest recommendation. It's a hit for all types of people, regardless of age. You can drive to the top and pay for parking, but you may still have to walk up the hill as parking is limited. The observatory is closed on Mondays. Make sure to check out the Tesla Coil show, which they turn on and shoot lightning out of, about once an hour.
Warner Bros. is the best tour, hands down. You first go into a room with the history of the studio. The tour groups are then brought into a theater for a quick video then it’s off to the lots, and sound stages. The last two areas are self-toured.
$29 per person. Plenty of restrooms and AC. Huge variety of botanical gardens and even has both Japanese and Chinese gardens. It also has a couple really nice gallery buildings for a nice museum feel. It takes a long time to see it all, give yourself at least 3 hours.
Note: this is a bit of a tourist trap. You should go, but just spend an hour or two there. One route is to go see the Chinese Theater and the hand/footprints outside, then check out the Hollywood/Highland mall area, go across the street to Disney’s flagship theater El Capitan, then go to the Roosevelt Hotel, Hollywood’s oldest that has multiple bars and restaurants including a great poolside bar in the back.
If you're heading to Santa Monica, Beverly Hills might be worth an afternoon. Park the car, walk down Rodeo Drive, look at the ridiculous stores, grab lunch anywhere from Spago to Subway, get back in the car, drive through a few of the neighborhoods North of Wilshire/Santa Monica, and call it a day.
Highly recommend going on days that aren't Friday, Saturday, or Sunday. Super Mario World is especially fun, but it will be crowded. There are Early Entry and VIP passes to help mitigate the crowds. Lastly, the butterbeer is delicious.