I love printing things from the Magic Artist website at disney.com:
http://disney.go.com/magicartist/index.html
They have all these wonderful things you can print out:
-calendar pages that you can use as a countdown calendar-- mark the days you'll be in Disney with a little Mickey head!
- paper dolls that you can cut out ahead of time and use on the plane
- personalized story books-- you enter child's name and some info and you can print out a book!
- coloring pages
- activity books
- games such as checkers
This site is so awesome. So many different characters area available! I love having the option to print things out as B&W or color to save on ink! Whatever you print out in B&W then also becomes a coloring activity!
I also second the scotch tape! I got this set from Disney that had different patterened rolls of tape and one dispenser. My DD loved using it and would just play for a long time. You have to be careful in case the cutting edge is metal. At the Dollar Store or craft store, you can find patterned tape in scrapbooking and craft sections.
Other things I've used with my DD3-- who has flown 6+ times since she was 18 months old:
- lollypops-- usally banned from home, can help with ears popping
- new book (lift the flap type or pop-up for extra interest!)
- my little pony (I bring along a bunch of butterfly hair clips and barrettes-- DD's old ones-- to use on the pony hair. She LOVES it)
- colored pens and paper
- fruit by the foot- it takes her a while to eat a whole roll-- chewy and helps with ears popping
- DVD player
- lacing cards
- construction paper (I found half-sheet sized pads at Dollar tree)
- stickers-- DD loves the foam ones, but be careful b/c they're usually super sticky! Plus you'll have a lot of backing papers to clean up.
- individual boxes of cereal-- DD loves to snack on cereal and by bringing the little boxes, I get a variety and don't have to worry about crushing it too much. Again, it takes a while to eat.
- iPod with her favorite songs-- behind the ear headphones worked best on DD. We couldn't use in-ear ones and found that the old-style ones that go above your head didn't adjust small enough to fit her.
All those things were sucessful in keeping DD busy. However we would do a different thing each time just to make sure she would be occupied. As she got older, she got much more restless and didn't nap as long.
We also always used her carseat and plan on doing so for our upcoming trip. We found it easier to keep her in her seat. The one time I flew w/out the carseat, I was flying alone so I thought it'd be easier not to use it and checked it instead. BAD idea! It was so simple for her to undo the airplane lap belt so she constantly wanted to get up and move around.
However, to carseat or not is a whole other thread.. sorry for digressing...
Have a great trip!
p.s- our flight time is normally 3 hours, but I have the unfortunate knack for picking flights that get delayed for hours at the airport.

So these tatics work equally well at the airport.