The models.CharField is a CharField representation of one of the choices. What you want is a set of choices. This doesn't seem to be implemented in django (yet).
You could use a many to many field for it, but that has the disadvantage that the choices have to be put in a database. If you want to use hard coded choices, this is probably not what you want.
There is a django snippet at http://djangosnippets.org/snippets/1200/ that does seem to solve your problem, by implementing a ModelField MultipleChoiceField
.