Some of my favorites :
crayola color wonder sets
paper and crayons
a few new toys to surprise and entertain (small cars or trucks that take up little space)
a small flashlight or something that lights up for when it is dark (try to find something that won't annoy other passengers)
ALWAYS have the must-have favorite toy (if your child has one).. for comfort
thin, lightweight paperback books to read
if you have an ipod, load it up with kid shows and make sure you have kid-safe/friendly earphones (the adult earbuds are painfulLY large and can be harmful) ILUV makes a white headphone that folds nicely and is noise-cancelling so the volume does not have to be so high (they are not so expensive at the PX)
snacks galore, especially if you have a picky eater .. the airlines are cutting corners everywhere and may only have peanuts and no pretzels for kids (happened to us recently). MAKE sure you are NOT trying something new .. not a time to find out your child is allergic to something!!
I carry a small shopping bag with handles that rolls up and fits in my pocket .. it is filled with toys and stuffed in my backpack. I pull it out after takeoff and hang it from the adjustment nob on my childs car seat so that it is easily accesible. When it is time to clean up, I sometimes have a second bag for the things that might need a cleaning(smeared with food or stepped on in the aisle ..)at the next stop.
For your return trip, ditch into Target and get a mini Doodle-Pro by Fisher Price, they clip on to your backpack for easy access while waiting in line at customs,etc
good luck and don't forget your sense of humor!!