I have a five year old boy, too, and here are the things he loves to do on planes. He doesn't have any electronic games and we don't take the portable DVD player on planes, but he manages to stay busy and happy.
-Listening to audio books on his MP3 player. We try to download a few new ones for him to listen to before each trip. Some of his favorites to listen to lately have been "The Remarkable Journey of Edward Tulane", "Peter Pan", any of the "Magic Treehouse" books, and the 'Binnucula" books. This keeps his quiet for pretty much the whole flight and in the airport.
-Like somebody else said, he loves drawing and writing in notebooks. We always have a spiral notebook and colored pencils with us. He hates coloring books...always has...but LOVES his notebook. He also loves stickers, so I try to have a few new packages of those for him to use in his notebook.
-I try to take a few special snacks...things he doesn't get very often at home....gummy fruit snacks, M&Ms, etc. Since they're a special treat, that helps keep him really happy!
Honestly, that's about all it takes for him.