The corn dog nuggets at Casey's Corner are really good! I usually have them with fries for a quick lunch. Not a big meal but enough to fill you for a little while. Casey's Corner is in Magic Kingdom towards the end of Main Street. It's the place with all the tables with striped(i think red and white) umbrellas
If you just want a quick snack there are stands all over the place at all the parks that sell ice cream, popcorn, soda, water, fruit, etc. For some reason, turkey legs have become really popular in Magic Kingdom lately....I've never eaten one of the ones there but they're huge and greasey and I get nauseous looking at them...but that's just me lol
I recommend getting Birnbaum's WDW dining guide. It's really helpful! They list every restaurant and snack place and give you an idea of what they serve and the price range