Fish tanks, computer equipment, lighting, heating pads, portable air conditioner, electric blankets, power tools, just on and on. 

I had a solid tenant that wanted a dishwasher.i knew they were going to be in town for 3 years (doctor in residency) and so offered to give them a portable dishwasher in exchange for signing a 3 year lease.

The units were utilities included, as the old building was not separately metered.What we initially did, was allowed tenants to get an air conditioner - either window or portable - and pay an additional $50/month.What started happening was, tenants would sneak in portable units, and run them after dark.  

I can tell you this this year I had a ac problem and while waiting for parts ( I fix my own units ) I ran a portable that I have as a emergency, well it was one, lol.

For instance, "I'm sorry, but our policy is that we don't allow portable washing machines in any of our units."

I don't care for portable space heating units in a rental so it would help if you could elaborate on what you mean by " not requiring them to buy a room heating unit'.

Rehab-based real estate investing is the only industry where you are frequently told you don't need to understand the main focus of your business to turn a tidy profit at it.Imagine you're sitting in an office and a salesman comes in and tells you there's huge money in portable toilets.

They gave a demonstration in our office a few months back and their furniture looks very good and extremely portable and easy to setup. 

Put portable storage or Conex/containers there. 

:This may sound silly, but have you considered placing a portable toilet on the property?