A nice sit down dinner (about $60-$70 or so if you splurge) is the California Grill in the Contemporary Resort. Call ahead and make reservations, and try to snag a window seat during the fireworks @ the MK. The view is spectacular---you can see almost the entire park and watch guests take the plunge down Splash Mtn. or go for a spin on the Astro Orbiter (I'm beginning to sound like a guidebook =b )...
Seriously, though, it's a nice place to eat.
If you plan to party all night (or at least until 2 AM) at Pleasure Island, might I suggest Portabello Yacht Club. It can get a bit crowded but still maintains a nice atmosphere, and is great if you like Italian food.
And last, but not least, if you like sushi - try Wolfgang Puck's. You can sit @ a regular table or at the sushi bar - be sure to ask to see their sushi menu though as they don't always hand it out with the regular one.
I hope this helps!
(Oh, and a final plug for Cosmic Ray's Starlight Cafe -- my favorite counter-service restaurant @ the MK!)