I'll add my two cents as well!

When are you going? Each season has different offerings, so check out the seasonal events and meet-and-greets in the program. For example, last year there were a few special villain meet-and-greets, as well as a special halloween miniparade, as well as others.
*I would recommend 2 days at least if you have the time. The must-sees are doable in one day, but you'll be rushing, and particularly in the Disneyland Park, there's a lot of atmosphere to soak in, so having a little more time would definitely help. *One reason I love DLP is that you don't have to plan everything months in advance. They still have the paper fastpass system, and you don't have to make dining reservations 6 months in advance.
* *Walt Disney Studios Park.** As others have said, this will probably be a disappointment to you. The two must-see attractions are Crush's Coaster and Ratatouille. I would recommend getting in line for Crush as soon as it opens (there are no fastpasses), maybe using the single rider line (that's a crapshoot because the cars hold 4 people), and then heading over to Ratatouille and getting in the single rider line. Or, you could run and get a fastpass for Ratatouille (there will probably be a line for the fastpass machines), and then head to Crush. Also check out Cinemagique (times vary throughout the day), maybe Tower of Terror (copy of Disneyland's version, I think, but it's one of my all-time favorites) which you can get fastpasses for), and maybe Rock'n'Rollercoaster. People complain about Toy Story Playland, but I think it's adorable and it has some pretty neat theming if you're a fan of Toy Story. Plus the Toy Soldiers Parachute Drop has some cool views of the park.
* Parc Disneyland. It's hard for me to make recommendations for this, because I had an annual pass over a four months period, so I would go like every weekend and just wander around and do whatever I felt like since there was no pressure. But, I have done every attraction in the park so I will give it a go.
Do Fantasyland early, because it closes an hour or two earlier than the rest of the park (u/fakeplasticmax said Adventureland does as well, but only parts of it do. I've gone on Pirates minutes before the park closes, but you just have to take a long way to get there).
(don't miss the castle - there's a walkthrough Sleeping Beauty attraction and the dragon's lair underneath)