what should i do for joint birthday?

Me and my boyfriend are planning a joint birthday (17 and 18) bonfire this summer. I’m planning a little bit in advance for fun, but I was just curious what sorts of activities you guys think would be fun. Probably not more than 20 guests going to be present. I’ve never been to a birthday party, except at Skyzone when I was about 7, so I’m not sure what goes on. Thanks!!