I want to make a surprise for my bestfriend and her boyfriend? Please read!!?

My best friend means the absoulte world to me. She's done so much for me. We've been through a lot together. This year is my senior prom and due to some things going on I couldn't afford the dress and such. Well she is letting me borrow her beautiful pink dress, jewerly, and shoes. Just so I can go to prom. As a way of thanking of her I want to do something nice for her and her boyfriend. He's a good guy I've met him and we even became friends. I just don't know what to do. We live by a park so I was thinking of packing them a pinic lunch and setting up with flowers and such. The reason why I want to do something nice for her and her boyfriend, is becuase her boyfriend works and goes to school. He doesn't have that much money so they can't always afford to go out. Also since they are both in school they don't get that much time together and I want them to be able to have that time together at no cost to either one of them. So anyways, do you think that's a good idea? Or do you have any other ideas? Thanks everyone!