California Grill is definately an extravagant treat - though you can skip the €300 Bottle Champagne to cut a corner or two
. Great view of the castle at night, elegant dining, yeah.
I'd also recommend at least checking out Walt's in the park, the themed dining rooms (to each of the DLP lands) is something you'll want to see.
You absolutely need reservations. There are kiosks in the park where you can make them, it should be one of your first tasks. If you don't you'll be waiting long into the night to get something to eat - and that includes counter service. Meals are long a liesurely, accept that, relax, and enjoy your dinners.