I agree with others, I wouldn't bother with the car seat on the plane. We also give our boys (2 and 3 years old) the window seat, they LOVE looking out the window. And that usually entertains them for a good portion of the flight.
We let the boys pick out a few of their favorite toys that they want to play with on the plane and then I pack a few surprises for them. I also make sure I have plenty of snacks that are easy for them to eat and get to so I know they can't fuss while on the plane that "it's not what they want"
As for the toys we have for the two, their back of toys usually include: the leaspster, coloring books, flashcards (my 3 year old likes to practice his letters all the time

), travel magna doodle, stickers, Disney Cars Matchbox cars and of course DVD's! Surprisingly this all fits in their little backpacks that they carry on their own. I don't bring anything that won't fit in these. They LOVE having their own backpacks and carrying their own "luggage" around, makes them feel grown up.
As for snacks I usually pack goldfish, pretzels, fruit snacks, animal crackers, cheez-its and some lollipops for take off and landing.
I have to say I am surprised each and every time how good they are when we fly, guess they think it's a special treat.