I just had a group of 20 teenagers over last night.
I barbecued hamburgers and hot dogs, and we had some side dishes also.
The bonfire was lit, and there was music.
Music, fire, and food are the keys to a great teenage party!
The kids sat around and talked and laughed.
They are cheerleaders (it was a cheerleader bonding barbecue) so they practiced a few stunts (with the cheer coaches okay) and just danced and goofed off.
Everyone had a lot of fun and really enjoyed themselves.
Our backyard is huge, so we had a lot of plastic chairs, a gazebo (nobody sat under it though) and the bonfire. Snacks were in the house.
It was in the evening (don't know when yours will be) form 7-10p, so the bonfire was a must-do (we have a fire pit)!
edited to add : At night, glow sticks are a hit! And teenagers don't really need a whole lot of activities - with them less parental involvement is best. They are pretty good at entertaining themselves.