You haven't given me any idea about where you live or your age (except you're a lot younger than I, like everyone else on this forum, hehe)
But here goes. Weather permitting, most girls would enjoy a picnic, as long as you do all the work. If you lived here (FL) I'd suggest the beach.A park might be more suitable where you are. Get some food she would enjoy, even KFC if she likes it, a nice blanket & tablecloth, flowers & vase-it doesn't have to be fancy, an old pretty jar would do- wine or sparkling cider if you're underage, maybe one of those citronella candles, you get the picture. Don't forget a CD player or something for background music. I'm sure other people will chime in with more (and better) ideas.