1. Head phones that go over my sons ears so he could listen to his shows without disturbing the other passengers.
2. My iPad which I downloaded a bunch of his favorite Disney Junior shows on. I made sure that they were new episodes that he hadn't seen yet so they would keep his attention. I bought them from iTunes where I also rented Madagascar 3 for him to watch. If you don't have an iPad I would recommended a portable DVD player, I bought a brand new one from a Pawn Shop a year and a half ago for $30 and it went everywhere with us. I have also seen them for sale at 2nd hand stores.
3. A new toy that I kept hidden until we were on the plane. I bought him a Hot Wheels Race And Play Car for the iPad.
4. As you can probably tell my son is obsessed with cars right now so I packed some of his favorites for him to play with.
5. I packed one of his cups for him to drink out of during take off and landing to help prevent ear pain from the change in altitude. I also packed a Mio so I could give the water I bought after we went through security a little flavor so he would actually drink it.
6. I brought his favorite blanket to make him feel more secure and of course keep him warm.
7. I also brought his favorite stuffed animal which could double as a pillow if he fell asleep.
8. I made sure to pack some of his favorite snacks
9. I packed a small coloring book with some of his favorite characters in it and some crayons in a zip lock bag, I only brought a few colors that way I wouldn't be searching the floor for a whole box of crayons after we landed.
10. I packed a few new books that I bought from The Dollar Tree, again I hid them until we were on the plane.
11. This is the outfit that I dressed my son in for the plane ride, I tried to make him as comfortable as possible. I also packed an extra pair of comfortable clothes in his backpack just in case.
12. My son wears Pull Up Training Pants but I put him in a night time diaper for the plane ride since they are extra absorbent and he isn't fully potty trained. And of course I packed extra night diapers in his backpack.
I hope this helps if you are planning on traveling with a toddler, whether it be on an airplane or on a road trip.