I'm quite industrious and like the concept of efficiency (that's not to say that I ever actually achieve it, but I do aspire to it!) and there are lots of things I like to do with a spare 10 minutes.
Food "research": Definitely my favourite 10 minute filler is flicking through cooking magazines, books and websites. I could look at food-related material for hours.
Housework: I actually made a list about 6 months ago called "Things you can do around the house in under 10 minutes". It was to encourage Gib to do more housework because he has this idea that housework takes an immense amount of time and therefore never has time for it. The list had things like taking out the rubbish, stacking/unstacking the dishwasher, putting on a load of washing, putting clothes away, tidying the desk. It was moderately successful, but I had to keep changing the colour of the sign so he would notice it. I'm always amazed at how much housework can be achieved in 10 minutes.
Stretching out: waiting for my lunch to finish in the microwave or for the kettle to boil, or when my head is aching from too much new information at work, I like to stretch. I'm all for incidental exercise, and stretching in my spare moments is so refreshing! There are a few that I can do that don't make me look like a complete idiot, such as discrete calf stretches, pec stretches, hip flexor stretches... Maybe, for ultimate efficiency, I could stretch and work at the same time?