There is not much good that Jem would like to remember. From chapter 21-27 we read of the loss of the case of Tom Robinson, the sworn hatred of Bob Ewell with respect to Atticus, and the Halloween costume party at school, where Scout is dressed like a ham. Life seems to go back to normal.
However, if you wanted to create a scrapbook of events as they unfolded, you would probably want to record a few important points.
First, you would want something to remember the trial. Perhaps you could write a note describing the outcome of the case and how Maycomb failed from a moral perspective by indicting an innocent man.
Second, you might want to record the indifference of most of the people in town. However, you would want to remember that there are good people that fight for justice like Atticus, Heck Tate, Miss Maudie, and Judge Robinson. This will help everyone to fight the good fight to the end, no matter how hard.
Third, you would probably want a piece of Scout's Halloween costume to remember both the depravity to which some people will go as well as the good fortune that allowed you and Scout to be alive.