There are shuttles offered by Mears (at least when i was last there). It was very nice. They pick you up at your resort in a van and then you meet up at a shopping center somewhere in Celebration and switch over to a bus. The bus takes you there and then sits and waits. If I remember correctly the bus leaves at either 4 or 5. I think we got there at around 10, so we get to spend all day there. NASA have different tours and movies and a whole lot of stuff to see. When I was there they had just started building something similar to Mission:Space so that might be something to look into.