We used the iPad and the stewardess told us to turn it down. It was on so low I couldn't even hear it! I had no idea you had to have headphones! Oops! So make sure to pack some. Btw, does anyone know some good kid headphones? I don't like the ones you have to stick inside your ear for a 3 year old. Thanks!!
We use the Kidz Gear wired headphones (easy to find on
Amazon) for both DD6 and DD2. It has an extra cable that dampens the volume, but you probably won't need that on the plane. They seem pretty comfortable and they are easy to adjust size-wise and volume-wise (it has its own separate control). And the most important part is that they stay on!!!
As for what fun things to pack for the airplane, I pack one or two post-it note pads... when DD6 was younger, she loved to peel them off and stick them everywhere and DD2 is showing the same tendency. Now, DD6 likes to use them to write little notes to us.
For our upcoming trip to CA, for DD2, I'm bringing a "book" of stickers from Michaels and paper... this will keep her busy for awhile.
For DD6, we pack a book, an activity book, a special pen and snacks and lollipops. DD6 loves the anticipation of finding out if I've snuck a surprise into her backpack.

Another great thing to include is the travel sized book of Where's Waldo... it's a collection of the Waldo books and it's smaller sized so it fits into the backpack. DD6 keeps herself busy with that one.
Another fun thing we sometimes do (if there is time), we let DD6 browse at the airport shops after we've gone through security and let her pick out a book or something similar.
We travel on flights between 3-6 hours at least twice a year and our most popular plane activities have been movies/apps on the iPad and coloring (love triangle crayons!). I try to find different coloring pages online and make my own coloring books. Another fun coloring option are the mini coloring sets that Crayola makes that are either princess themed or Cars or Toy Story themed (at least those are the ones I've seen) and it comes with small coloring pages (fits great on the tray) and four mini markers.
HTH!