Here's what I typically carry in my backpack as a college student:
Pens/highlighters/pencils This is a bit of an obvious one, but I did want to mention that it's useful to always be prepared with an assortment of colored pens and highlighters for color coding notes at the library.
Planner Some weeks I need my planner more than others, but it's best to carry it around all the time. In class, I jot down assignments when the teacher mentions them so I don't forget and in the library, I like to refer to what I have to do and what's due (here's how I organize my planner).
Body spray Often I go from dance right to class and it's essential to have a body spray (I like this Ralph Lauren one). It's good to have in your bag just in case, especially in the warmer months when you can work up a sweat simply walking to class.
Lip balm I hate the feeling of chapped lips and it can be distracting when you're trying to focus in class or at the library. Having a lip balm on hand is always a good idea. I love the fun flavors of eos, plus they work really well!
Headphones I usually don't listen to music while I study, but I do like to have my headphones in while walking to class. Other tech essentials to have on hand are a phone and laptop charger.
Laptop I love my Macbook Air because it's so light to carry around. A snap-on protective case keeps it clean and safe from scratches.
Hand sanitizer This is a must, especially with all the germs at schools and college buildings. I like this all-natural sanitizing spray from The Honest Company.
Kind bar It's always good to have a snack on hand just in case you find yourself rushing to class with no time to eat. Kind bars are all natural and healthful, with lots of protein and fiber to make me feel full. I love all the different flavors that you can choose from as well, from sweet to savory.
Water bottle I find bottled water to be so wasteful and I avoid buying it whenever possible. This light, reusable water bottle from Bobble with a built-in water filter is a favorite of mine.
Hand lotion Moisturizing is especially key for the cold winter months when my skin gets so dry. It can even hurt if it's too dry, which can become distracting from studying or class. A creamy lotion with a pleasant scent can sometimes be such a mood boost!
Tissues I am always sneezing because I have bad allergies. I also think buying tissues is oddly fun because of all the fun geometric patterns on boxes and packages. A to-go pack of Kleenex is definitely a backpack necessity.
Of course you'll also need your notebooks/binders/folders in your backpack as well!
I recently replaced my old JanSport backpack with this Patagonia backpack. Most people at my school have The North Face bags and I didn't want mine to be too hard to find in the mass of backpacks outside the dining hall! I've found the Patagonia to be a great size -- not too many compartments or pockets, just a simple, mid-sized backpack with room for all the necessities.
(On a side note, I love supporting Patagonia because they have a great sustainability policy and will repair your products if they ever fall apart. I'm sure I'll have this backpack for a very long time!)