My kink is sounding. I don't actually care much who does it to whom, so I'll throw out a few options for you to choose from --
1. OT8: Haruhi and Renge apply them to all six guys
2. Tetsuya and Kasanoda
3. Any subset of the OT7, really, whatever you think would be most fun to write...
NOTE: Sounding is inserting long, thin things into the penis. It's one of those that probably qualifies as extreme kink, and I suspect it might be difficult to put into any kind of believable Ouran context. I would kind of hate to expose these characters to the extreme BDSM of most of the stories with sounding that I've read. Here's the urbandictionary (http://www.urbandictionary.com/define.php?term=Sounding) definition, and here's the wikipedia entry (http://en.wikipedia.org/wiki/Urethral_Play), which has a good deal of info.
Reading what I wrote in my request, I think I may have gotten a little over-excited when writing that... *g* I still think Tetsuya/Kasanoda might work, and possibly also Renge teaching Haruhi on some host of the writer's choice...
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examples of sounding fic - I'm not wholeheartedly comfortable with using these for reference for the host club boys, because -- *wraps arms around innocent ouran boys protectively* -- but here's what I've got. All extremely, extremely NC-17. You've been warned.
Sara's Song, Samurai Champloo, Jin/Mugen, by Laura Bryannan Ginger is the main kink in this story, sounding is kind of like icing on the cake. Um. Search for "grace yours with a little gift" to zip straight to it, it's near the end. http://www.homestar.org/bryannan/champloo/sara3.htm
Corvus Fallere, HP, Severus Snape/Harry Potter, by Mouse This is Harry and Severus + some fantasy creatures which are half crow and half human. This story is rough, I'm actually not so much for breaking of slaves who don't want to be broken, but the sounding bit is omghot. Skip down to the last scene on this page, it starts with "Hot scaly hands." http://www.the-archive.net/viewstory.php?sid=696&i=1
Blood for a Parched Tongue, HP, Severus/Sirius, by Amanuensis This is Severus taking revenge on Sirius, and all kinds of toys and kink. Sounding is used as torture here, with something with a sharp tip -- look for "slender stylet." http://www.amanuensis1.com/bloodforaparchedtongue.html