A phone charger
Not to sound like a major millennial, but your phone is your life source. As a woman, it’s a safety tool. You can call an Uber when you feel unsafe taking the subway at a certain hour. You can look up directions. You can look up directions that take you through safe neighborhoods. You can double check the password you emailed yourself to get into a building. Everything is in your phone so if it dies when you’re out and about…good luck finding a stranger who will let you use theirs. They have places to be. You need an emergency phone charger.