Hi, John!
What a great plan you have so far! You're going to love Old Key West Resort, too; it's one of my family's favorite places to call "home" while vacationing in Walt Disney World.
For that last full day in Walt Disney World, I have several suggestions. If anyone in your party is a fan of Star Wars, then don't miss
Star Wars Weekends at Disney's Hollywood Studios. Star Wars Weekends run Fridays, Saturdays and Sundays from May 17 through June 9, 2013, so you'll be able to enjoy everything this special event has to offer on May 24th. (Trust me, your experience that day in HS will be entirely different than your experience on Thursday. Events, parades, dance parties, celebrity autographs and all-around Star Wars-themed fun is packed into every corner of the park.
If none of you are fans of Star Wars, then consider heading back to either Epcot or Magic Kingdom for a second visit, as these are the two parks that you simply will not be able to fully tour in just one day. Magic Kingdom Park is open from 9 a.m. to 11 p.m. that day, with Epcot offering evening Extra Magic Hours, resulting in the exact same schedule as MK: 9 a.m. to 11 p.m. This means that you'll have plenty of time to enjoy either park that day, squeezing every last drop of magic out of your park ticket, too. Play it by ear, once you've enjoyed your first trip to both parks, then choose based on which park has more attractions that you simply didn't get to enjoy...or would love to try a second time. Stay as late as you can, too, so that you can fully enjoy the nighttime extravaganzas offered at either park. What a way to cap off your trip!
Send more questions as you think of them, John; we love to help!