If you do send food, make sure to send enough for your camper's entire cabin. Newspapers and magazines. If your child reads a paper daily, clip out his or her favorite parts and send them off every few days. Kids love getting the sports or comics in the mail. Forward magazines as well, especially for longer camp stays.
Books are another good care package idea.
MadLibs are really popular for older children and young adolescents. Batteries and other expendable items are good filler items depending on what your child brought to camp. Small toys are okay, but make sure your send items that will not break lights, windows, or foreheads when they inevitably get thrown in the cabin. Baseball or other trading cards go over well. Many companies offer pre-made care packages.