Cookies (like No-bake cookies, those are the shit)
Potato Salad (Mrs. Gerry's if you have that)
Fresh fruits (you can sweetly feed them to each other while sitting under the tree on the hill at sunset)
Some other dessert if she hates cookies (how can you hate No-bake cookies???)
Go with the foods you know she likes. If you have a company called Edible Arrangements around you order one of the fruit baskets, they are awesome to have for a picnic.
Quick foods are the best way to go though, peperoni and cheese platters are good, chips and dip if you want, premade sandwiches are good but some can be quite mushy by the time you eat them so it may be best to make sandwiches yourself ahead of time the way she would like them.