Pick a theme, get with the right AMA sig if applicable, and promote it to other clubs. It's nice to have an idea of how many people are coming so you can get enough volunteers to keep it all organized. One suggestion I would make is to be specific on the theme of the event. "Just come fly" can be done at anyone's local field, so why would they travel to yours? Give people a reason to indulge in a special interest like pre-WWII planes, jets, aerobatics, etc.