Of course, there is a limit to what you can take with you on a teaching assignment; nevertheless, some items are basic.
It might go without saying, but you should carry a pen. School personnel should not have to supply you with one when you sign your pay voucher. The pen speaks to your preparedness for the entire job.
If you catch colds easily, a hand sanitizer might be another basic item for you to carry. Along the same lines, you should have a handkerchief: not every classroom has an ample supply of tissues sitting on the teacher's desk.
Additionally, a flashlight, perhaps a small one on a key chain, will light your way in dimly lit corridors that could be cluttered with miscellaneous school supplies. You don't know what shady recesses you could find yourself travelling.
Then, for some substitute teachers, an odor-eliminating, disinfectant air spray may be an additional “must carry”. For a number of reasons, a classroom may have odors that the regular classroom teacher has become accustomed to; you, however, might find the smells overwhelming.
Again, some substitute teachers find their assignments have periods when the job is not demanding: they are assigned to be with a single student while he/she works on an assignment, the regular classroom teacher has a planning period and the substitute teacher is unassigned, etc. At these times, a book for leisure reading, a current newspaper, a crossword puzzle might become an essential to shun boredom and, still, set an example to students of positive ways to fill idle moments. Playing a game on your cell may not set the same example.
Finally, you must have some emergency lessons with you. If you substitute K-12, general lessons should be prepared that could cover chunks of the spectrum (K-2, 3-5, 6-9, 10-12, for instance). Think in terms of general information everyone should know: safety, manners, cultural practices.
Many of these items can be carried quite discreetly in a purse, tote or briefcase. For those of you who are constantly on the go to one substitute teaching job after another, a small file on wheels might be a good investment. Covering an underlying file space, some of these small files also have a tray to outfit with note pads, markers, pens/pencils and can be pulled with a telescoping handle or carried with an alternate handle.
But, whether you fill your pockets or arrive with a file, make your day by pre-thinking the needs of the day.