How to change the default Activity dashlet to include activities other than "scheduled"?
You would need to change the code. But there is another way to get the Activities on your dashboard.
Go to Reports, select Contact Reports and then Activities. If you then set all filters as you wish and tick the 'Available for Dashboard' option, and save the report as "My Desired Activities" (or any other name :-)) you should have a new dashlet available for your dashboard with your settings for activities?