What is the best and easiest wall covering to use?

My tenants put up blue board in the home they are renting from us and we are now being forced by the Board of Health to "finish the walls in a manner in which they are smooth and washable." What would be the best and least expensive "stucco, putty, plaster, etc" that we could use? Is there something out there where we would not have to also paint? Thanks so much!!!