Clip4
Touchdown! Greaser!
You can instruct without any medical if the person receiving instruction can act as PIC. Other than that, you can give instruction in an aircraft that you are qualified to act as PIC. For example, if you have a CFI and are flying as a Sport Pilot, you can give flight instruction towards a Sport, Private, Commercial, etc., as long as it is in a Light Sport Airplane.
Sure, you can do all that, but the opportunities for all that a quite narrow.