It's very important to have something to suck and swallow during take off and (especially) landing - it can be as much as 20-30 mins of ear popping on the way down.
Talking about what you see when you take off and land - small houses, cars, fields, then up through the white clouds - talking about what clouds are made of. MAKE SURE YOU GET A WINDOW SEAT
There are some great magical Kipper episodes which happen in the clouds - have them in the DVD player? Several Richard Scarry DVD moments in airplanes too.
The tray of airplane food can itself provide literally minutes of innocent entertainment for children and adults alike
Friendly crew might offer them a tour of the cockpit to meet the pilot?
At the airport: escalators and lifts can be fun. Weighing child on an unused check-in carousel? PRetending to drive one of those little electric car things for disabled people (when not in use). Pushing a trolley around? [Although if child rides on trolley, the security staff may come and get officious] Check in, and then go for a walk outside the airport until it's time to go through security? Lots of colouring things? Sticker books?
Wait until the flight is actually called before going to the departure gate. The departure lounge often has amusement arcades for children, some entertainments in which are free; might be worth budgeting $20 for rides? Looking out of the departure lounge windows at the planes and staircases on lorries and baggage trains etc might also be fun