The smaller the storage area the more money people generally get.  So that got me thinking about areas of a house that can't be rented to a tenant.  Like if there are no windows, no air circulation, no fire exits, etc.  Most houses have a few areas like this.  Some people don't need a lot of storage space but they may want to keep some things private or simply away from where they are presently living.  Your space could be their solution.  All it needs is a solid door and a lock.  If one wanted to take security further one could put video surveillance in that area that is motion detected, posted online for the renter with text notification.  What do you think?  

And what about being a mailbox for people?  This way there is no Post Office box, no mailbox, not even a suite #.  Just a name at a house address.  The problem with this is probably neighbors seeing people come and go.  They wouldn't like that.  Of course the person using your mail delivery service would be notified when there is something there and who its from, minimizing traffic.