A link to Princeton's majors and certificates:
Princeton University - Departments & Programs
Hopefully you're interested in one of those! You'll notice that only one of the certificates is offered as a major, and this is because a certificate is different from a minor: a certificate supplements your major, and when you write your senior thesis you have to incorporate aspects of your certificate into the thesis. They're a sort of specialization, if you will, and for the most part people obtain certificates (there's no limit as to many you can get, I believe) in fields closely related to their area of study; however, I heard of people doing a physics major with a certificate in theater. Their thesis was a play about physics! The faculty will try to do everything it can to accommodate even the most unique interests, though it's not always easy.