Crayons/coloring books, activity books and puzzles are great - they are flat so they slide into carry-ons easily and don't weigh much. Kids can also do them on the tray table. Go to Target and comb through the magazine/book section. They have TONS of cool coloring books that come with their own crayons or other coloring instruments, like water pens (you fill it with water and then when the water comes out the tip, it colors the special pages - no actual ink). My kids love knowing they get new books for trips. And you don't have to worry about keeping track of a whole pack of crayons this way.
Also, if you do not have a TAG reader by Leap Frog, they are awesome for this. You'd be investing a little bit of money, but they are not super expensive and will continue to be used when you get home. And they are awesome for teaching kids to read. The reader itself is a pen-like device and when your child touches it to the reader books, it reads to them. The great thing about the pen - there is a plug for headphones. Genius! On a trip to Hawaii, my 3 year old daughter not only sat on the plane for 8 hours and read books by herself this way, but she also would sit in her bed and "read" while we showered and got ready for dinner, etc. Very useful tool and also educational. Plus they have all kinds of books so no matter what your kids are into, there is something for everyone - Dr. Suess, Disney princess stories, Dora and Diego...you name it.
Hope this helps! Have a great trip!!