Epcot is nice for a Sunday activity if your stay extends over the weekend and you only want to break the Sabbath in as mild a way as possible. Animal Kingdom is interesting and has some fun rides if you want a really good zoo experience with a Disney twist. Hollywood Studios is really fun and it would probably be my suggestion of where you should go if you haven't been there, and you want something different. It has some great shows and some of the best rides, and the lines are marginally shorter than at Magic Kingdom, probably. But if I were just there for one day I would do Magic Kingdom. Splash Mountain for the win.