I don't allow my family to bring electronics on vacation, so I've always relied on craft store supplies to keep them busy on the plane. Here are some ideas my kids liked when they were younger:
*an assortment of pipe cleaners
*flat buttons and beads (things that won't roll)
*an hourglass-style egg timer
Your kids can make games (tic-tac-toe, pig) out of these supplies. They can also create hats and jewelry. My kids loved having "art museum" races. We'd turn the timer, someone would name an object, character, or animal, and we'd race to build our best representation of it. When the timer ran out, we'd compare creations.
*small
Lego sets
*minature etch-a-sketches
*dollar store cards and games (I found Disney themed Old-maids and magnetic checkers boards there)
*blank books...encourage the kids to journal their trip, starting with the plane ride. Provide colored pencils so they can draw the inside of the plane, the view from the window, etc.
Stationery: Encourage them to use their time on the plane to compose a letter to their favorite character. Little ones can draw pictures. Take these letters along to the park and have your kiddos deliver them in person.
A small favorite toy from home, to create their own "The Adventures of..." book...and a couple of disposable cameras per kid. Encourage your kiddo to carry the camera and toy in their fanny pack, and take photos of the toy several times each day, with a fun new background each time. When you get home, you can use these to create a book about your trip. Begin the photos on the plane ride!
Books...I always have my kids bring along two or three books. When they were small, I helped them choose favorite books that would be comforting to have in a strange new place. As they got older, I allowed them to purchase new books that would keep them occupied on the plane.