We just went on a 14 hour day flight with our 19 month old. Surprisingly it wasn't too bad (I had imagined much worse things happening). My DS is very active I thought he would not be entertained at all.
I took lots of snacks, sandwiches, etc.
I had various new cheap toys that I would bring out every so often. Mine loves cars and animals so that was fairly easy.
The big winner for us was stickers. I bought lots of rolls of cheap cheap stickers, shiney ones, etc and spent ages sticking them and unsticking them into sticker books, on his hands, face, my face, on the seat, etc.
We also walked him up and down the plane and visited the stewards and they seemed to enjoy it too ... I guess as long as you don't get in the way too much.
Enjoy! We are doing an 8 hour flight on Monday, this time it's only me (pregnant) and DS so I might not be very happy on Tuesday. Luckily we're off to my parents and I hope they will be delighted to see DGS