Maybe everyone could bring a favorite storybook. Mary mentioned costumes -- I remember my son going to a birthday party where everyone was to come as a favorite character from a book.
Also, most guys love gadgets -- how about having a camera to take digital pictures, or instant Polaroids, or even video, of the festivities? You could do some great "daddy-daughter" pictures.
Maybe hot tea and iced tea could be served along with sandwiches and cookies. Mini-hamburgers or cheeseburgers might be fun.
I'm sure you'll get some great ideas. What a fun party to plan!