Write a note to the teacher about how the day went. I've never had the opportunity to call in for a substitute (yet) but I'm sure the teachers appreciate it. My "notes" often turn into a novel and I feel somewhat bad about this but hey, I have big printing.
Sometimes I'll break up the day in morning and afternoon, sometime I will break it up for each class so that the teacher knows exactly what we did in each class, other times it will be more of a "the class was great and everything got done" kind of note. I usually write the first half of the note at lunch so that I don't forget about the morning during the afternoon.
I also try not to write too many bad things. Sometimes the teacher asks for a list of names of kids that could have been better and then I'll give them the list. But this rarely happens. They have to be really bad to get their name in the note. I think in all 5 years of me subbing I've only written a completely horrible note a couple of times. The last time I did, the teacher e-mailed me and apologized. She also told me she made her class write letters of apology to me but I was too scared to ever return to her class to get those letters.
Don't forget to write about the good things. Lots of amazing things happen every day. I like to write about funny moments (I hope the teachers get a little laugh about them) and write down names of anyone who was extra helpful that day. Like parents, teachers don't just want to hear about all the horrible things their class does when they are away. Tell them some awesome things about their class in the note too.