Are there any activities which are treated as neither a supply of goods nor a supply of services?

Yes. Schedule-III of the model GST law lists certain activities such as (i) services by an employee to the employer in the course of or in relation to his employment, (ii) services by any Court or Tribunal established under any law, (iii) functions performed by members of Parliament, State Legislatures, members of the local authorities, Constitutional functionaries (iv) services of funeral, burial, crematorium or mortuary and (v) sale of land and (vi), actionable claims other than lottery, betting and gambling shall be treated neither a supply of goods or supply of services.