Kind of a tie for me:
- Most of my trips are offsite and I typically go to Epcot on getaway day, then drive to the airport from there, simply because I find it the easiest park to get to the car without transportation (trams, monorails, etc.). That last walk out of Epcot is torture. Vacation ends when I walk under the ball and head to the car. I try not to look back.
- This one's been mentioned. Walking thru MCO, watching the happy arrivals coming in, knowing you're on your way out. (Of course, the reverse of this is an early highlight of the trip)