A lot of companies offer a "care package" service which is convenient but it takes the fun out of it for the kids. Every gets the same box of pretzels and peanut butter crackers at the same time, and it's not very personal. So, although it's more work, I'd send personalized and unique packages.
The kids get plenty of junk food in vending machines and from other kids on the "package service" plan, so I'd go for the healthier stuff. My son made his own granola bars during high school, so I continued those in college - any recipe with oats, flax seed, fruit like raisins/currants, honey/agave and peanut or cashew butter will work. Make sure the roommate doesn't have a nut butter allergy. They pack well because they aren't round like cookies, so they don't slide around and break.
Despite my above comment, kids do like homemade cookies too! Bubblewrap is the best packing material to prevent too much sliding around, I found. Also seasonal stuff - I sent Halloween candy and pumpkin muffins, and at Chanukah I sent 8 little packages of traditional foods/treats.
Sometimes kids need to laugh - so don't be afraid to send silly stuff like those fake vampire fangs at Halloween, bubble solution while the weather is still warm, and any other favorites from when your kid was little. When it gets cold, try lip balm and hand warmers, an extra pair of gloves, a silly winter hat (my son had a whole collection of things with antlers or a fake zebra tail).
Small gift cards are fun too. Do enclose something like photos relevant to the time of year - photo of their first snow fort when the first storm is expected at school, Halloween costumes, making Valentine's cards for Grandma, etc.