Idea's for my 17 birthday?

Hey! Okay so march 5 i will be celebrating my 17th b- day. I was thinking about inviting at least 20 friends. If the weather is good that day since NY has such a bipolar weather we could all go on a picnic at central park. Then we could go boating and maybe hit Times square to watch a movie and take lots of pics. This sounds all good and affordable but I need a backup plan. I'm scared it might snow during that week, What else can I do? Ideas please!! :)