I have come to realize that I am providing a service to many teachers in need of personal and professional time off from their teaching. I now have a high regard for those in charge of finding and organizing substitute teachers. They do an excellent job.
It has been enriching observing other disciplines material and I've been lucky a few times to actually help some students with their math. There is some uncertainty to deal with not knowing where you maybe from day to day or what kind of day it may be, but you get accustomed to it.