There is nothing wrong with carrying a lunch pack. I often have very long lectures (3+ hours) that hit around dinner time which are followed by club meetings I run. I also have diabetes, so I guess carrying a lunchpack and preventing a low is better than seeming like "an adult". When I bought my pack I opted to purchase a child's lunch box because it was compact and cute. I find it easy to strap it to my backpack or even to toss into my purse (I say purse, but lets be honest, it is more like a tote bag ).
Either way, this is what works for me. If you know you are going to be on campus for a while, and are going to be hungry but don't want to pack a lunch, try searching for healthy options around your campus.