We always bring home some fudge from the Main Street Confectionery (tip: bring a tupperware containter with you to pack it in your carry on bag and keep it fresh) it is such a nice souvenir to enjoy while relaxing the day after your trip.
My favorites to enjoy in the parks: fresh popcorn on Main Street, dole whips, the sundaes from Mrs Potts are the best, Donald's Breakfast at Animal Kingdom, Wine Tasting in World Showcase, Starring Rolls bakery at MGM and if it's cold when you go the Ghiradelli chocolate shop at Downtown Disney has the absolute best hot chocolate.