1) Chalk. Kids love to color and outside is no exception. They'll quickly beautify any concrete structure and probably even give your outdoor furniture a face lift. (I always buy the washable chalk, but I wonder if that even matters. Isn't all chalk washable?)
2) Bubbles. (Word in the park is the bubbles from Gymboree are the bomb.)
3) Balls. Can you ever really have enough? Probably not. I can't count the number of balls that my kids have lobbed in various neighbors' yards never to be seen again. It seems there's always a shortage, which, of course, invariably leads to screaming and tears. (The most popular ball we have I bought at the dollar store, so there's really no excuse not to stock up.)
Woe to the mommy who is caught without the essentials.