Hi Kristin,
What fun having a first trip to the Walt Disney World Resorts! I love Disney World in the fall especially with Mickey's Not So Scary Halloween Party and Epcot's International Food & Wine Festival. Take away the humidity and I would never leave:)
While I have never been to the Kennedy Space Center, I can tell you it it will take you about 60 minutes to drive to the Space Center. I would think you need a good 4-6 hours to tour this facility if not longer.
You will need to be at the Orlando International Airport at 4:00 pm to check in for your returning flight so I think it is tough to go there on your last day. In addition, you never know about traffic.
If you do not have a ticket for the last day, you can shop Downtown Disney or relax at your resorts pool. If you want to get off property, Gatorland is a legend in Orlando and is on the way to the airport. You could pick up a rental car from Disney property and then return it to the airport. You would have more than enough time to feed the gators and then head home.
Wishing you a magical vacation!
Beth