As a sub, I absolutely, positively cannot live without my cell phone. It's how I get jobs. Now, mine is not fancy, but it doesn't have to be. It just has to answer and make calls.
The second thing I can't live without is my alarm clock. How else would I be able to get up at the crack of dawn to make it to a school across town to sub?
Finally, I must have my sub bag filled with all the things I will need during the day.
What do I carry? Everyday, I have my coffee. I would be nothing without that. Ha! I also bring my own pens, pencils, sticky notes, etc.
Classroom management items are a must for any sub. I carry some Caught Being Good forms, my punch card positive behavior system, and my treasure bag.
I also have business items. My calendar is important because often teachers stop me in the hall or lounge and ask if I am available for another day. My calendar allows me to answer and schedule more jobs right then and there. I always have my handy dandy notebook... just in case. And I make sure to have a Daily Summary Form to report to the teacher how the day went.
Finally, I make sure to have on hand some extra activities and plans for those just-in-case moments. There are those times when plans don't take as long as expected or the teacher had an emergency and was unable to leave to plans. If you have meaningful things for the students to do, you will never have to panic. I love to have a picture book or two with a couple of graphic organizers that would work for any book. Laura Candler has an amazing book filled with graphic organizers.
If you need emergency sub plans, check out my store. I have a set of free plans for grades K-5.
I have a set of Just Add Paper plans for grades K-5 for those true emergencies... you don't even need to make copies for these plans.
And there is a set of CCSS Emergency Sub Plans for grades K-4 that have a day's worth of lessons based on one picture book and aligned with the Common Core.