(1) How about working as a tutor or teaching assistant at college? Even if the content is not necessarily aviation related, you have the opportunity to polish your instructional proficiency.
(2) Observe other instructional flights from the back seat. Anticipate how you might handle various situations.
(3) Review instrument procedures with Microsoft Flight Simulator. Find new approaches that you have not performed. Explain what you are doing as you "fly," even if no one is there to listen.
(4) Chair fly. It is FREE and a good practices even for experienced pilots.
(5) Review and update your CFI notebook. Could the lessons use some polish? Is the content well organized?
(6) Review the FAA publications: AIM, Airplane Flying Handbook, Pilot's Handbook of Aeronautical Knowledge, Instrument Flying Handbook, Instrument Procedures Handbook.
(7) Find an area of which you could benefit from further understanding. Meteorology? Engine operations? Aerodynamics? etc.