Nearly any novel source of extra income you can imagine is possible if you’re willing to look for it and make it happen. Some students have bought and resold textbooks, and I had a side hustle in my first summer writing study guides and test questions online. Arts and crafts are another option. The blog Personal Finance for Ph.D.s is a good resource for not only increasing your income but also reducing your expenditures.
The most professionally relevant opportunities are often those through the university, and for those, it pays (literally) to pay close attention to messages from the graduate program faculty and staff. For example, I can recall off the top of my head two summer instructor positions (one with the physics department and one with the engineering department for a physics course) and two internship-like positions (one reviewing tech proposals with the university’s patent office and one with the University Fellowships Office) sent out by physics program administrators this year. Of course, you can always search for opportunities on your own as well at psu.jobs, where internal Penn State jobs are posted.
Penn State offers the following additional information: