In my wife's teacher contract it states she may have "other duties as assigned". Can this include duties that are outside of the school day?
Example: work in consession stand at athletic events on non school days or in the evenings.

Generally speaking, yes. However, not having read the contract in its entirety, we wouldn't know for sure. My understanding that this is not uncommon in education.

Is she a member of a union?

Actually, I'd be surprised if this was permitted by contract. Except where after school activities are a given (like the sports coach showing up for practices and games) usually, the contract ends at a set time of day. Obviously teachers grade papers and such outside the hours they are in the building, but they can not be required to take on other duties outside their teaching obligations. Many volunteer to. Some districts do pay extra for such things. Either her union or contract should explain what extra duties can be required.