lol, we did this recently and came up with 100 ideas for 'c' alone.
the best ones that made it were:
chippendales - big fake chests and bunny ears from a joke shop
chirstmas pud - home made, based around a hula hoop and lots of material
crack addict - bit dark that one, lots of makeup
cowboy - easy, good fun and you get to carry a gun and wear silly boots
caveman/woman - simple, just need to find the material
can - was a giant can of fosters, took a lot of effort, but was well worth it