Opportunities are time sensitive (The opportunity is for an advertisement that will run on April 1st) I need a workflow that will change the Opportunity to "closed - Lost" 5 days after the Opportunity "Close Date". I also need to build a report that shows all Opportunities closed by this workflow.
Use a time-based workflow. The criteria can be something like:
Stage Name not equal to Closed - Won
Add a time-based field update set to 5 days after Close Date.
If you want to be able to identify records that were closed by this process automatically, you'll want a custom field to track this, because workflow updates and manual user edits are indistinguishable otherwise.
I'd add a checkbox with a name similar to "Automatically Expired", and have the workflow field update trigger from the same workflow rule as above.