Shameless plug - If you're like me and like to keep things simple, check out the shirts in the Reboot Whovian Teespring store for easy "costumes." Even better, you can get use out of them on normal days, too.
Below are a few recommendations. Check out the store for all available shirts, styles, and colors. http://teespring.com/stores/rebootwhovian
Your costume is the 12th Doctor commenting on his own costume. Throw a long black coat over it to add some splash without much more effort:
For something topical, give a nod to the Doctor's most recent costume addition - the sonic sunglasses:
If you can't decide which Doctor to represent, why not all of them:
On the topic of previous Doctors, remember when the 10th Doctor became human? Why can't we just be a human for Halloween:
Even the 11th Doctor tried to fit in with the rest of us by putting on a jersey and playing football.
For a bit of mystery and mischief, no one is better than River Song.
If you prefer creepy and scary, try a hand mine.
Don't think we've forgotten about the kiddies and babies.
In any case, have a wonderful Who-lloween! Although, when have we ever needed a holiday to celebrate Doctor Who?