Okay, so the first thing I'm seeing is that we've got three days where there probably won't be much magic or wishing. Karyn hasn't really seen any "extra" effects from the wishes, and Jon is kind of on auto-pilot for the duration of the weekend. There are a few things you could do:
- Depict a Wedding
Personally, if I were writing I'd avoid this. It would effectively end up being a regular wedding, with only the reader knowing something is weird. This can get old pretty quickly, unless you have some specific idea for interesting things to happen. My instinct is that any interesting things would create a new storyline rather than contributing to the current one, however. Personally, I'd show a few highlights from the weekend for Jon in Karyn's role, maybe some moments where physical anatomy causes some hiccups that nobody notices, but generally skip the wedding and the weekend.
- Focus on Karyn
Karyn is not on auto-pilot, so you can give her a chance to explore a bit. Maybe let her have some observations about Jon's relationship with other friends of his or his family. Maybe she changes Jon's relationship with some of these people. What if Zoe goes "hey, Jooooooon, let me punk you out with a goth makeover," and where Jon would normally shoot her down immediately, Karyn is actually kind of down with it. Maybe Mikey challenges Karyn to a game of Super Smash Bros, and this being not one of Karyn's favorite games, Mikey beats his "brother" for the first time in his life? What if Jon's mom is watching a Hallmark movie, and is surprised to find her son climbing up on the sofa with her and getting really into it? What if Karyn finds herself cornered by a bunch of football players and they start making fun of "Jon the virgin," so Karyn "proves" that he isn't by telling them stuff they don't know about how sex works for women, which unexpectedly impresses them and raises the real Jon in their esteem?
- Change Karyn
What if she starts acting like Jon in the same way that Jon has been acting like her?
- Sit on the Toilet
Errr... I mean
- Timeskip
Just start the next chapter as Jon is regaining control of himself. Give a light review of the events of the weekend, but keep general plotty things sparse. Even without the auto-Karyn wish, Jon was steering towards subconsciously becoming Karyn. I'd exacerbate this. Even more pronoun slip-ups, even more taking ownership of Karyn-things. Maybe this starts to concern Karyn. Maybe this even makes her jealous. Maybe they both realize that things have gone too far and have a serious moment. Maybe they start getting confused about who the "real" Jon and "real" Karyn are, and have an existential crisis that can only be solved by waiting out the wish or making a new one.
All of this is just off-the-cuff. Does any of that speak to you?
