Add a programmable thermostat.21.

In addition, he told me that there is currently only one thermostat for the entire house, and that it's difficult and expensive to zone on a steam boiler.

Yesterday I received a text stating a tenant did not have heat. allegedly the pipes were banging but no heat was produced so they shut the thermostat off.  

This comes with an Aerus Air Scrubber and an Ecobee smart thermostat and sensor to ensure I can keep the basement and upstairs temps nearer to each other and at least some cooking smells don't permeate.

The triplex has one thermostat for all 3 units.

And good for her if that is the case).If you're going to be in that top 1%, and have the personality disposition to do either (I do not, hate driving, weak sense of aesthetic, tell me the kitchen "feels too cold" and my response would be to turn up the thermostat), Realtors at that level out-earn loan originators. 

We came in painted the walls and stair rail, all of the doors, and cabinets, changed all of the light fixtures, added ceiling fans, and upgraded the thermostats to wi-fi ones instead of the basic ones.

OP hasn't said if they're even on a separate thermostat.

I would not feel comfortable with tenants being responsible for anything more than basic maintenance (ie. filter change, replace thermostat batteries).

I use the Honeywell 9000 series WiFi thermostats to remotely change temperature settings.I would look up houses on Homeaway.Com that have bookings in the slow hot period and see what amenities they have that are attracting people and see if you could incorporate any of these into your property.Maybe you could also donate a summer week to a charity and use that as a tax deduction?