Genderbent Panty and Stocking
is always fun. :P It's so popular you can find wigs of it.
Fem!Kyon (AKA Kyonko)
(the picture's a good laugh if you've read/seen the series :P) would actually probably be something I'd be willing to do, considering I already have the costume and if I just grew out my hair it'd be perfect. :P
And then there's genderbent VOCALOIDs. My boyfriend was talking about being genderbent Miku
with one of our friends as Rin...I kinda laughed at him. ^^;