high-intensity productivity

Most people will probably agree, that scientific research is a more mind-intensive activity than other jobs and that a researcher has probably less productive time per day available than most other people, may they do physical work or clerical work.
Once we have admitted to ourselves that we can only give the best to our research for two or three or maybe four hours a day; and once we have admitted that all this aimless internet-browsing, and facebook-scourging for the rest of the day is indeed useless, but can not be replaced by just being hard against ourselves and forcing ourselves to do more work. Once we gained that insight, we can grow on it and ask: what other useful activity could we do to fill the remaining eight hours of day? What activity can give us energy to work on research the next day? What activity keeps us happy, does not leave us with bad feelings about not working on research and at the same time does not consume our mind so much that it distracts us from research?
I have found that even such a simple pass-time as sailing can distract too much from research work, when I start thinking of sailing moves during lone moments at night, instead of my work.
As grad students we are TAing, attending presentations, discussing our work and theirs with others, organizing things, ...
Isn't it a big part of the problem of productivity to schedule those activities so that they become a useful time-filler for the time that's non-productive in research?