Personally I found six hours instructing in a day plus pushing the aircraft around and three hours driving to and from the airfield quite a long and tiring day - in no way could it be classed as 'relaxation' or 'rest' either in reality or in terms of the meaning and intent of the airline definition of a 'rest day'.
Not s***e in my opinion. Which is why I didn't instruct when I flew with airlines who forbade it and out of common sense when I was flying long hours commercially and was tired after a demanding duty period even when I was permitted.
I have always had respect for my employer's wishes and the common sense not to unecessarily fly when tired and so possibly compromise the safety of my crew and passengers.
Maybe you're different? In which case it might be you who ends up in the s***e?!
KR
FOK