I you want people to use cars less and the buses more, you put up airport parking prices until they are unaffordable, and have a free bus service. Then you deal with the irate backlash.
If the airport buses are almost empty, you cut out every second one, and educate the passengers to plan ahead and get an erlier one. That will cause a flood of irate phone calls from them too, complaining that they had to wait hours in the airport because they were too early, and others from passengers who "didn't know" so missed a flight. - however, that would increase sales of food and drink in the concourse.
It is all swings and roundabouts -but you could always install a whole lot of play equipment in the main hall, for adults as well as children. Gambling machines, what the butler saw - and how about a flume from the top floor to a pool in the centre of the ground floor hall? Charge for those too. Take RyanAir as a model.