>"Oh, well in that case, let's party! I wanna see just how much everyone can drink before passing out!"
>Nudge Suika lightly and whisper; "5 Yen says Cirno passes out first."
Well, there's always the option of Mima getting up to shenanigans for no other reason than boredom...And ending up potentially causing something that could end up almost as bad.
(Cue Rumia going "This again?!")
Or you could go with those templates everyone says is forbidden.
...Idea!
Mima, wanting to alleviate her boredom, decides to start another incident.
Yuuka, also hella bored, decides to start one of her own at, ironically, the same time.
And Sanae, on Orders from Kanako or Suwako, is deployed to 'resolve' the incident, while laying the foundation for more shenanigans from Suwako.
And somewhere along the way, Team 9 gets wrapped up in the thick of it(Rumia: "Is that so?!"). Cue our Idiot Hero getting on the job, like a true Incident Solver.
With a surprise appearance from Medicine!
> "Yeah, Rumia's apparently over a hundred years old or something." Reimu said, before adding, "I'm 18, so's Marisa, I think, you wouldn't get an streight, honest answer out of her anyway."
> "I can read your minds to make sure... especially Marisa" Satori said.
> "I'm over Sixty, if that flower thingy happening twice in my life was about sixty years or something." Cirno said.
> You notice Sakuya vanished as soon as Satori mentioned she could read minds to find out ages.
> "Uh... I think I'm over 18." Wriggle and Mystcia both say. Satori sighs, and looks through their minds for experiances. "Yeah, Mystcia and Wriggle met Cirno about two years after the flower thing Cirno was going on about."
> "By age do you mean production date or coming to life date?" Kogasa asked.
> "You're an umbrella you can't get drunk anyway." Sanae said.
> "Oh yeah!" Kogasa says.
> "Sanae... you're not eighteen..." Reimu said.
> "I know... I was waiting to say bye! I don't really want to go past the bridge youkai alone anyway, she's creepy." Sanae said.
> The only one who turned out underage besides Sanae was Utusho, who, having only just became anything more than a Hell Crow, turned out to be three years old.
> "Well that explains a lot." Reimu said as Satori announced that, sending Utusho back to tend to the Blazing Fires.
---
> The party begins. You quickly discover you like this Sake stuff.
> "You have Yen? I only have what I can sneak from Reimu's donation box... which is nothing, because it's always empty. I wager Marisa fires a Master Spark before Cirno passes out." Sukia said.
> "That's not even a bet Sukia..." Yuugi says.
---
[I have a few ideas jotted down, I hoped to do something with some of the characters who were not involved in either Rumia Quest so far. Of course, about half the characters you mentioned haven't been involved :V. I will say I hope to involve the Three Fairies at some point, even as comic releif [Do we need more? YOU CAN NEVER HAVE ENOUGH].]