In about 2 months we will be going on a trip which involves a 5 hour flight. Our 2-year-old is typically very active and I'm really worried that he will have a hard time on the flight.
What are some good techniques for making sure he stays happy?
I've actually had to deal with virtually this exact situation a few times over the past year or so. We're in California, my family is in DC (5 hour flight). My son is almost 3, my daughter is 1.
As with all things in dealing with small children, it's really a game of distractions. So the best way to prepare is to be well equipped with a full arsenal of distractions.
One of the greatest tips that was given to us is new toys... These will generally be your secret weapon. When you anticipate your child growing bored, break out a new toy for him to play with. I'm going to assume that you are familiar with your son's tastes, so with any luck he'll be entranced with his new toy for 30-60 minutes (or more, if you're lucky). Try to avoid giving him a toy during a tantrum, or you could be inadvertently rewarding bad behavior.
Here's some toys that were great for my son...
Toy taxi. Although, really any vehicle will do. On another trip, we gave him a toy mail truck... he still plays with it at home!
Magnetic Tin. It's a metal tin, about the size of a book, and opens up to reveal a landscape background image. It comes with a bunch of magnetic vehicles, people, props, etc. There are tons of them out there, with different themes (farm, city, dinosaurs, etc)... This one was a huge hit with our son!
Water Wow! "paint" book. This thing is brilliant. There is no actual paint, you just fill the brush with water. Every picture is a thin white surface overlaying a full-color picture... When the kid gets the page wet, the white becomes transparent, giving the feeling of "painting". We've got plenty of Melissa & Doug toys, but this is probably my favorite (um, I mean my son's favorite).
Most importantly, make sure to keep these new toys hidden until the moment is right! And only leak them out one at a time... You'll want to milk each of these distractions for as long as possible.
Good luck, and enjoy your trip!