well what are you doing already?
you could spend the last day at the marketplace at downtown disney and shop for all your gifts and souvineers. There are tons of outlet malls near by.
as for the extra day, aside from the four parks, make sure you do a waterpark. if youre doing one already you may find you want to go back. typhoon lagoon was one of our best days. were thinking about doing a trip down just for the waterparks this summer.
go resort exploring. or take a liesurely morning and about mid afternoon start on a progressive dinner. go to the dinning baord and see who has the best appitizer and go there, then go somewhere else for the main course and somewhere else for dessert.....
you could leave disney and go to cocoa beach or the space center.