You have two good options... maybe three... if you want counter service. The Columbia Harbor House is much like a Captain D's but with a nice atmosphere. It also features a (usually) quiet upstairs with a great view over Liberty Square and Frontierland. Cosmic Ray's offers a nice selection and is your standard fast food fare, but features an animatronic show ala Chuck E. Cheese, and an unlimited condiment / garnish bar. A third option is El Pirata y el Perico, but it is only open seasonly to accomodate larger number of guests - if it is open, it offers an excellent taco salad with unlimited extras to put on the salad. A final mention is Casey's Corner, which offers hotdogs... the only reason I mention this counter service is because it can be ideal at night when watching the fireworks or parade since you can grab a table outside and they have free cheese dip (it's basically nacho cheese) to go along with your fries or dog.
For table service, the Liberty Tree Inn is a very nice spot. The Crystal Palace is a character buffet, but is still very nice as well. Both places will require reservations. You can already mark off the restaurant in Cinderella's Castle... and regardless, you're probably not missing much.
Hope that helps - make sure to stop by Adventureland and grab a Dole Whip!
BL