We did this after our cruise - because we got a ride to the port by family pre-cruise we saved money overall by renting a car to get us back to the airport after for our 4:00pm flight. Since we have WDW APs, we decided to go back to MK for the half day(in hindsight, picking any park BUT MK would have been smarter, thanks to the boat/monorail eating up so much time).
We did the express walk off to make full use of our time, and I highly recommend it for this scenario. We had our last breakfast at Cabanas and faced no line in the atrium walking off the ship. Based on my photo timestamps, we left the atrium around 7:20am and were waiting for our rental car shuttle at 7:30am. We went with Budget because of a great coupon code we found, and even with other families on the shuttle with us we were still in our rental car by 8:00am and parked our car promptly at the TTC by 9:00am. We headed out of the park around 12:45pm to ensure we'd have enough time to take the monorail back to the car and take the car back to the airport in order to wait in line at security (again, we probably could have squeezed another half hour/hour of park time total had we not gone to MK - but sentimentality wins out!)
To us, this was the absolute perfect way to end the trip, and while I wouldn't recommend it if you don't have APs, I loved it!