We just took a trip on a plane with a preschooler and toddler and found these toys great:
The memory game has a variety of cards, and is a nice two-player game.
The travel tangoes are small and can be made into many shapes, and the magnetic pieces are harder to lose.
Wiki Stix are fun to make into shapes and letters, though plain pipe cleaners would do the same. The wax on these are nice and tactile, though.
The mini slinky was good for both kids.
The farm animals were fun for the toddler, who is learning to talk, but makes some great animal noises.