For myself, I bring a good book. I usually don't get to read it much though because I'm too busy entertaining the kids. For the kids, I bring:
Portable DVD player
Assortment of Disney DVD's - Target carries a line of Disney cartoon classics that are perfect for this!
Handheld video games - we have Leapster and Pixter
Color Wonder markers & coloring book - Disney themed of course!
Books to read
Birnbaums WDW for Kids by Kids - DD8's favorite book
Disney Stickers - DD3 likes to stick them on herself and other people (my apologies to whoever may sit within her reach)
Snacks - fruit snacks and raisins are great because they are small and chewy since DD3 is too little for gum during ascent/descent
Wow, that sounds like a lot of stuff, but it really isn't that much. Usually I I will toss in a couple of surprises for the kids in my carryon, such as the stickers, coloring book & snacks, and maybe a new DVD or game for their handheld. I stock up on these things when they are on sale. The
Disney Store right now has stationery sets on clearance for $2.99, and small digital cameras for $15.99. They also have character-themed twist-up crayons for I think $2.99.
I hope I gave you some good ideas! I wasn't sure if you were talking about fun things for yourself or for kids, I know most of my ideas are for kids. I'm usually either too busy taking care of the kids or I entertain myself just by looking out the window & people-watching. DD3 also loves to read "the airplane book", aka the safety manual on the plane. She loves the pictures of the people sliding out the emergency exits.