Unique Graduation Party Ideas?

My son and daughter are graduating high school and I'm throwing them a grad party. There are about 250-300 people coming. We have been to so many parties and they are all relatively the same. We have all decided to have a "beach theme". It's not going to be a kids party beach them, kind of more wedding like (shells, candles, lanterns, ocean blues) that kind of thing. What I'm not sure how to do is make it unique enough. We have the theme, just not what to do! What kind of food? How to display their HS memories? Activities for friends and family? Thank for your help in advance!