Well, we just talked about nature in an earlier question today, when we talked about the dependent arising nature of our food. Looking into the dependent arising nature of everything is very helpful. I'm going to give one meditation at the end of this period to do that. Another thing is that any time you see any external decay of anything - decaying wood, decaying buildings, or something decaying on the path. etc. -- it can be helpful to bring up impermanence and reflect on what it used to be, what it is now, and what it will eventually become. That is seeing the impermanence of things.