P.S.
for the plane:
small cans of playdoh w/a few popsicle sticks
triangle crayons & a tiny notebook
Wiki Sticks
magnets (games or puzzles)
post it notes of all sizes (they can stick on their seats and around them)
Rubiks cube (key chain size)
portable DVD w/headphones - pull out during the last hour of your flight
Pack everything in separate ziplock bags and pull them out every 30mins or according to however long you are on the plane. When they are through, all you do is throw it back in the bag and pull out the next bag - no shuffling through your carryon in a tight space!
for the beach: after spending a fortune on fancy beach toys, I decided to bring a stack of disposable plastic cups and a package of different size popsicle sticks for my son b/c I was so sick of all that equipment taking up room. It was so much easier to pack too. I threw in a few plastic beach cars and trucks and he spent HOURS playing w/them w/the cups and sticks. I would bring a hat you can tie under their chin or fits snug enough so it won't blow away. And kiddie sun glasses...kid's eyes can't filter UV rays like adults can.