The items I would include in the bag relate to themes and ideas that Ben discovers in the course of Deadline.
A padlock is one item I would put in the bag. It locks things up and cannot be opened without a key. This object symbolizes how Ben wants to lock up the truth. He does not want people to know this detail so he "locks" it up and throws away the key. The lock can also represent how people like Dallas and Rudy also "lock" their secrets away from everyone else. The lock is what guides the entire narrative and its key is the truth.
I would include a copy of On Death and Dying by Dr. Elisabeth Kubler-Ross. It would be important for Ben to read a book on how to cope with the reality of death. While Ben does make profound insights on the nature of truth, he does not seem to effectively process what death is. The book would be an effective tool to help Ben understand something very difficult.
A football would be included to symbolize Ben's desire to be the best football player Trout High has ever seen. The challenges he faces on the football field are similar to, though far less than, the adversary he is facing in his terminal diagnosis. I would also include a pair of running shoes to symbolize how Ben left cross country for football. In some respects, he is "running" out of time and the shoes could work on this symbolic level, as well.