If you're talking something like an egg sandwich or breakfast platter, you'd be better off at your resort.
There are lots of places that you can grab a muffin and coffee or OJ at the parks, if simple is what you're looking for.
At MK - The Sunshine Terrace, behind Alladins ride, has muffins and juice. You could of course go to the Main Street Bakery, if you don't mind waiting in line.
At HS - The Writer's Stop, across form Star Tours, has coffee (some fancier things like espresso) and muffins.
Epcot - quite a few places have breakfast items, like the food court by Soarin'.
At AK - there is a bakery that's just inside Tusker House that has pasteries and coffee.
For value, I'd say grab something at your resort. Sometimes I go down to the food court early and bring the stuff back so people can munch while they get ready for a day in the parks.
