I replaced the carpet in the bedrooms with a luxury vinyl flooring from a local surplus warehouse store.

HVAC tech thinks the simplest path is to remove the oldest big furnace and replace it with a new small furnace for one upstairs unit, install a second new small furnace for another upstairs unit, then use the newer big furnace to heat the main floor. 

They've been dressed up over the years with new roofs, vinyl sizing, replacement windows, etc. but it doesn't change that they're structurally inferior to conventional stick-built houses of the same age in other areas.

She is asking for a month to month lease so she can jump ship before it sales.I have offered to find a replacement tenant while she looks for a new place.

One person finally called back today - City Engineer. 

If you only have one door generating income and a tenant leaves for whatever reason, then that property is considered 100% vacant until you find a replacement. 

Obviously it depends on the property, so some areas I’m thinking can be improved are window replacements, exterior painting or interior painting.

For example, on the places I self manage, if there's a toilet flapper that needs to be replaced, I can do it myself for $15 parts + my time.

One thing I know is that I am not looking to replace my current occupation.

@Christian Mani as others have said, help find a replacement tenant.