An updated version of this script with more bells and whistles can be found here.
If you have an outline with only a simple list of single action to-do items (i.e, no headlines with child nodes) and an Archive headline at the very end of your outline, this script does the following:
Choose "todo" from your Scripts menu to add an empty checkbox when you first enter the action item.
When you've completed the item, choose "todo" to add a checkmark to the headline.
If you don't want to see the completed item anymore, choose "todo" to make the headline a child of the Archive headline.
If your headline has any other icon value associated with it (e.g., ), "todo" will do absolutely nothing to it.
(If you do not see a github embed above, please refresh this page or get the raw code here.)
Your outline would look something like this:
incomplete to-do item
completed to-do item
If you expand the Archive headline, you will see your completed items.