Picnic right?... technically meaning you don't have to have a real meal. If you know her favorite fruits, I'd recommend you make a fruit salad with of course multiple fruits. I'd try exotic ones, kiwi, mango, passion fruit, oranges, bananas, etc...
I dunno how she feels about alcohol, but you could also purchase some "wine coolers" or just get fruit juice. If you want a real meal... call her up and ask her if she likes deli meat... figure out which ones she likes, go to your nearest grocery store and place an order for a small platter of sub sandwiches. For dessert... I find it quite common that women love Cheese cake... just plain cheese cake alone will make them happy.
Keep it simple, if God wills it, it'll be a beautiful day and you guys can enjoy eating outside in the backyard. Man to man... try to please her every which way, please her vision, her taste, her emotions, and her sense of touch/feeling. Be soft, respectful, and romantic; hand feed her (fruits), sweet talk her, and always keep eye contact when you speak to her and also add a few compliments here and there.
You'll be set homie

God darn it... I just realized it already happened. Bah! Well next time :P