Before the birth of my 3rd child, I traveled with my two children, by myself, ages 2-4 1/2 years old. Everytime I have flown with them, they receive a new book, and a small toy. (My 4 1/2 year old likes Polly Pockets, so I would buy her a new one, and my 2 year old likes Thomas, so I would buy him a new train.) Each child had their own roll-backpack that would get packed with crayons/markers/pencils and paper or coloring book, their new book, their new toy, and a couple more small non-noise making toys (Barbie, Polly Pocket, cars, etc). I also found a small lunch box to help hold their smaller toys that I put in the backpack. For my older one, she helps pick out the toys she wants to bring. The new toy and book are not 'revealed' until we are on the plane (packaging taken off). I would also pack their favorite doll and some various snacks. Finally, I allow my children to stand up whenever the seat belt sign is off.
Whenever I fly with them without my husband, I arrive at the airport 2 hours before scheduled to take off. After we go through security, I encourage them to walk all the way, even stopping at stores along the way. They basically run around the terminal until we are ready to load, so they're tired and ready to sit when the plane is ready to take off.