Hello there, @julieack!
We always leave on a late flight. I second @DizFinder’s recommendation to take private transpo to the airport. The bell desk can hold your luggage (at least they do at BWI and YC) and you just go back to pick it up and call a car.
We also travel light, so don’t usually have bags to check- that’s also a major de-stressor. Have your boarding passes ready to go. We use the airlines’ apps and ask for push notifications in case there is something going on with the flight. Also we make use of whatever TSA fast lanes we’re eligible for.
We pick a park we can walk to (for us it’s DHS or EPCOT) ride all morning and then have a late lunch there and walk back. Maybe hang around the pool or do something on the Boardwalk- anyway, something close to the luggage so nothing prevents us from leaving at the appointed time.
One piece of advice is don’t schedule anything big for that morning, like a character breakfast. It is easier to have a relaxed, perfect last morning without that fuss and the chance that something will go wrong with a big event you’ve been looking forward to. It is a perfect time to poke around and see the little things you may have missed, soak in the Disney aura, and start thinking about your next trip!