Taking Disney transportation back to the airport isn’t going to go as quickly as you hope. First off since you don’t have the earliest plane you will not be the first set of people off the ship to go through customs after you pick up your luggage. Second you’re going to be dropped off at ticketing not baggage so you’re going to have to traverse the concourse with your luggage and children. Then when you finally get to the car rental area, after an assortment of elevators, escalators and long hallways, you will be able to walk through the parking garage and pick up your car unless you have to take a shuttle to an offsite facility to pick up your car which is often the case for the lowest price rentals(add 30 minutes). So half hour to load the bus, one hour to drive to the airport, half hour to walk and rent car(Unless there’s a line which in the morning which is not out of reason). Load your luggage in the car because you can’t leave your luggage at the airport, but remember you’re going to have to get to the airport 2 1/2 hours early so you’ll need to leave with enough time to be at the airport at 4:30 to return the car.
LEGO land is not advisable it’s a much bigger distance than you think. And Orlando traffic can get ugly.
At Disney springs you would be able to set up that Aqua car ride, take the boat ride from one end of the shopping district to the other, take in a movie, see Disney’s large toy shops, the Lego toy shop, Gardideli‘s chocolate shop, Earl of Sandwich, The Disney Christmas store and maybe you could do all of the things I listed for Disney springs, but I don’t think so.
Look at a map for the downtown area now called Disney springs. Then anytime in the future, when you have extra time before flight you’ll volunteer to spend your day there.