-ipad with kid friendly headphones
-sticker or magnet books
-a few match box cars/happy meal type toys that if get lost don't matter
-snacks. Something you can dole out a small piece at a time.
-at that age, the window shade on the plane seriously bought us about 10-15 minutes of time

-looking out the window....seriously, don't overlook this. There are so many things to look at on the ground at the airport. Play I spy, watch the luggage, etc. On take off, watch the buildings and cars. Same on landing. Save all the "stuff" for when there is nothing to look at outside.
-more so on the way home, looking through pictures on the camera or watching trip videos
-games on the cell phone (since he is NEVER allowed to touch it otherwise)
I travelled a lot with my DS8 as a baby/toddler and now with DS20 months. Honestly, the less you can take the better. I just can't believe the amount of stuff people bring on planes. With a baby/toddler, keep it to a bare minimum and check as much as you can. It is stressful enough you don't want to be worrying about tons of stuff.