I'd agree with a bacchanalia and just naked bodies everywhere. Hell, I might go all-out synaesthetic with my narration at the height of things, just to say how overwhelming all of it is. In terms of specific character beats, I have some ideas:
> Chloe and Gavin have sex together, each others' firsts. It's as much about passion as it is about feeling they should do it, since Gavin's gone down on her and now they have a weird tension between them. But maybe it's not satisfactory, or seeing other people get freaky revs them up again, so they go on to have sex with other people. Threads set after the orgy explore whether they really want a relationship or something casual or to never mention it again.
> Kendra both lusts after Shawn and hates the way he treats her. As the night goes on, Shawn pisses her off so much that she decides to pick a guy (maybe Kurt, since he's already been flirting with her) and, as the night continues, couple with him at the start of the orgy (possibly starting the orgy?). Shawn doesn't like it when other guys try to take what's his, though, so he joins in, the two guys using Kendra together.
> Peter has a lot of pent-up frustration from his breakup with Hannah, and Taylor is just frustrated in general. The two of them jump each other because while just friends, they trust each other enough to try something more without it getting weird. Except they end up in a position where they're face-to-face with Zack and Hannah for maximum awkwardness on all sides. Whoever gets to that one can decide how they resolve the situation, both in the moment and in the aftermath.
> Kurt's been admiring Jennifer's body since they came to the lake as kids. He hits on her, and he's old enough and handsome enough that this time (you know it's not the first he's done this) she agrees. Jennifer is an even better lay than he ever dreamed, but he's no slouch in the sack either, pulling out all his tricks. Kurt wants to make her beg for him, and she's not gonna let a man so much younger ravish her without working for it.