-Some sort of fluids: bring extra water. You can make an iced tea or lemonade as well.
-Sandwiches are easy to make. Just go to your local deli/bakery and buy some good stuff. If you're very lazy, Whole Foods has a variety of good sandwiches.
-Fruits: wash some grapes or strawberries.
-Cheese: Brie or whatever you prefer. How can you not like brie!?
-Easy to eat stuff.
-Sometimes, I'll bring some wine. Depending on where you picnic, you may want to be more discreet about this.
Oh, and don't forget to bring your girlfriend.