When I leave my apartment I try and always carry these six things:
3) Passport - including Visa and Immigration papers
4) Metro Card
5) Student ID
6) Small Notebook including a map
Number 1 and 2 are pretty easy - I typically carry them at home.
The Visa is permanently affixed to my Passport and I keep this information along w/ my immigration card inside of a money belt - carrying a Passport is fairly common practice throughout Europe, but it is just a pain because they are so large.
The Metro Card I keep in a hard plastic sleeve that I typically use to protect baseball cards - my hobby finally comes through for something useful.
I travel with my Student ID even when not going to school because it is good for Student Discounts throughout the City especially at Museums and other tourist locations.
And I keep the map in case of some sort of emergency where I get dropped off at the wrong metro stop or some other craziness happens.
My most pressing issue right now is for one of the above items that I do not yet have - My Student ID. Billie Jo told me to call a person I have not yet met to talk to the guard if I am stops. The person is named Jon Smith - sounds like she is making this up.
Thankfully Jon answers the Phone and he talks to the guard. The Guard never smiles - nor will he remember me on subsequent trips - he pushes a button and the turnstile unlocks.
I am off to class.