Depends where they live. In my area, I would have to make sure they don't catch a cold, so being lightly dressed is not an option.

I would probably go for something classy. If it is a woman aged 18-30, probably as a form of Disney Snow White

and for a guy something like a Hunchback of Notre Dame

Honestly speaking i wouldnt make that bet because the prize doesn't give me anything i want.

Annabelle and Chucky.
Danny and Sandy
The Flinstone lady or man in the cartoon.

Make them go as Randy from a christmas story

Conan the Barbarian or Red Sonja

How about a full body suit that is painted to make them look naked.

Little rascals could be fun.