Our 2 year old flew for the first time at the beginning of this month. I was worried about keeping her entertained. I bought her a new rolling backpack with Nemo on it and made sure she didn't see it until we left. In the backpack, I put the following (none of which she had seen before):
12 Little Golden Disney books that MIL had dug up for us
Cinderella Color Wonder book
6 Lacing Cards in a ziploc bag
Wooden Bear puzzle (like a paper doll, but with wooden pieces)
Pipe Cleaners
Crayons and Mickey coloring book
Reusable sticker board with stickers
Earth's Best breakfast bars
Lollipops for take off/landing
We had direct flights with little wait in the airports and about a 2 1/2 -3 hour flight. Including the flight there and back, all we really used were the Disney books!
She STILL has not seen the Colorwonder book OR the Bear puzzle OR the Pipe Cleaners OR the Sticker Book OR the Coloring book! Basically, she just wanted one of us to be reading the Disney books to her, or she occupied herself by dozing (we had early morning/evening flights) or by eating the snack provided by the airline!
I do second the suggestion to bring a straw/sippy cup as our airline (Delta) did not even have regular straws we could use - fortunately I did have a sippy stashed in her diaper bag. Instead of the lollipops (which she had never had before) we ended up eating a Clementine for take-off on the way there and just drinking water from her sippy cup for the other take-off and two landings. She had no problems with any of them, and does have a low level of fluid in her left ear pretty much all the time. I was surprised and pleased with how well the flights went.
My DD tends to be pretty good about sitting quietly though, so really I should have known that I didn't need to bring much to keep her entertained. She was pretty excited to have her own bag though, and did pull it throught the airport, so that was helpful. We did not bring her car seat and never felt like we wish we had it with us. She sat fine on the airplane, but again we usually don't have a problem getting her to sit for a long period of time.
Good luck and have a great trip!