Fun-A-Day idea #26:

Find your grade school or high school yearbook, and write letters to your former classmates that you will (probably) never send.

Thank someone for standing up for you in the cafeteria that one day.  Tell someone how great you thought they were, in case they didn't know it then.  Tell the class bully that you're sorry they were so angry, and that you hope they sorted things out.  Thank the teacher who made a difference in your life (that's one to actually send).  And then let it all go.