Become a member of UNIFEM
For only $15, you can become a member of the New York chapter of UNIFEM, the United Nations Women's Development Fund. More information here.
They have some great events coming up, including a huge career fair for those interested in public service (October 10 at New School University), film screenings on international issues and how they affect women and of course, the annual Walk to End Violence Against Women and Girls.
All good causes, and it's cheap and easy to join.