Welcome back Ansam! You will certainly be off the ship by 9:00 a.m. at the latest, which leaves you with quite a bit of time until you would need to check in for your 5:00 p.m. flight. Because you'll have luggage with you, you might consider booking a rental car. This will be the least expensive way to travel from the port to Downtown Disney then to the airport. Check out Port Canaveral's website for the different car rental companies at the port area and decide which one is best for you.
Be sure to print directions at home so you can familiarize yourself with the way back. It takes anywhere from 45 minutes to just over an hour to get from the port to the Walt Disney World resort, and about half hour from the resort area to the airport, so plan your time accordingly. There are a few toll stops, so be sure to have quarters available so you don't have to wait in the longer credit card or "need change" lines at the tolls.
There is so much to do at
Downtown Disney. You might even make advanced dining reservations at one of the restaurants there to enjoy a final Disney lunch before heading back to the airport. Plan to be at the airport about two hours before your flight, as security lines can get long there. You won't have a ton of time at Downtown Disney, but a few fun hours there are much nicer than a long day sitting around the airport!
Have fun, Ansam! Let me know how else I can help you!