The first flight with our 18 month old son, I brought along the doodlepro, stickers, color wonders, etc. He wasn't old enough to know what to do with stickers. The next flight (age 2), I brought along a disposable placemat and put it on the tray thing and he stuck stickers all over it.
The color wonders didn't work out so well for us, because I don't like picking the markers up off the floor.
We considered bringing along a portable DVD player, but decided against it since he protests so loudly when it is turned off. I didn't want to hassle with his crying when the attendants ask us to turn it off.
The best thing we brought was cookies! We brought some homebaked ones, also some small packages of mini oreos and mini nutterbutters. Those were all a hit. We even gave a packet to a little boy crying nearby (with his mommy's approval).
Also I stopped at our local
Disney store for a couple little toys. The first flight he got a little Nemo phone. The second flight he got a Buzz Lightyear thing that looks sort of like a blackberry. Those occupied his time for quite a while. The toys talk, but no one could hear them once the engines started.