With my 4 boys they each get their own backpack but only
half full and the small stuff in ziplocs. Each usually has:
two books (short reads or coloring)
extra socks
extra underwear
extra t shirt
extra shorts
Gameboy and two games
A few wipes
Chapstick
A couple snacks
Headphones
Why the clothes? Well for one thing my kids are finalists
in the Worlds Sloppiest Eating with Shirt as Napkin contest.
But we have also had our luggage go off in a different direction
twice (so far).
As others stated, slip on shoes and clothes in layers.
But most important is Benadryl, my boys have ear problems
and it helps to keep the eustachian tube clear to cut
down on the pressure problems (sudafed for me).
Oh yeah, Jetblue whenever possible, and we avoid
the bigger airports (oakland instead of San Francisco,
Long Beach instead of LAX or Orange County).