This should be no problem at all. Leave the car parked and go enjoy the park. Be sure to leave your parking slip so it's visable through the windshield, just as you would do on any day.
I've used this method many times since I live less than a five hour drive from WDW and often want to use about half of a day to visit a park before driving home. Express checkout automatically checks you out so you don't need to take any action to check out; but you will want to put your luggage in your trunk before taking the bus to a theme park, because checkout time is probably going to pass before you get back to your car in the afternoon.
Enjoy.