80-100 hours really isn't a lot to dive into a whole new field. I was gonna suggest something in quantum information but that field is incredibly broad on it's own. If you're interested in theory in grad school you could get an early start on using Feynman diagrams, whether it be fundamental QED or a many body physics related topic. I really loved Mattuck's book for first introduction to them, he makes it about as easy as can be to learn the basics.