Zokin - cleaning the floor
We always clean/mop/polish the floor before training. This is partly because it is a big part of Japanese culture, but also because it is good exercise for the lower body and for balance.
Cleaning the floor by hand in such a careful way really helps you to feel like the dojo belongs to you and that the floor is very important. You literally and metaphorically become close to the floor!
When the local church youth group has had Friday pizza night and left ground in pineapple pieces and ham all over the floor (like there was this morning!) then a really good scrub is needed anyway.
Naoki Eiga used it as a way to go back to basics and reconstruct his kendo from scratch.