And, by "a plan".... you mean something for you two to do or...?
In my opinion you should prob have some ideas, but dont have it set in stone unless youre sure you both want to do it.
I dont know what you guys are like when youre together, but if you dont have any ideas then you run the risk of spending the whole night asking her, "so what do you want to do?" and her replying "i dont mind, what do you want to do" etc etc
Not that im saying that will happen or anything...but as the boy scouts say...be prepared!