•••16 birthday party ideas?

Ideas for a sixteenth birthday. Not too ott and won't be all alcohol I live near the beach but in England so weather isn't reliable birthday in September but can have party in August maybe Mixed group of friends half from an all girls school and half from my local youth group so that's mixed gender. The groups have a few people who know each other but most don't so I need them to mingle My idea = BBQ at beach with friends just volleyball and swim in the sea etc. So what are your ideas wet weather an good weather and how to get them to mingle. Please don't say cinema or meal that's too expensive My friends will all be 15 who go as I am old for my year except one 17 year old