Provided you can supply the appropriate amount of venitlation, you could keeping anything from a cornsnake or smaller in there. I personally would keep a cornsnake in a larger cage but keep in mind that most of the thousands of cornsnakes produced in this country every year were produced by adults kept in sweater boxes.
For a North American species I really like the Bairds Ratsnake. There are also a lot of Eurasian species that are dropping in price, are small, and make good pets.
Trinket Snakes, Diones Ratsnake, Twin-Spotted Ratsnakes, etc. to name a few.