This property was relatively turn-key, but I've added some amenities to make it even more attractive to young professional renters (furnishing the common areas with relatively nice furniture such as a big screen TV mounted on the wall, added a Ring Doorbell/a SimpliSafe Home Security system/individual locks on each bedroom door to make my tenants/roommates feel extra safe, etc.)

+1 if you can add in gas from the efficiency perspective, there are wall mounted propane heaters and may companies would lease the propane tank as well. 

I got lucky awhile back and bought a house this spring that in only a few weekends I had ready and rented it for exactly what I thought i could and it is motoring right along. 

Multifamily costs can mount that tenants do not pay for reducing your return.

I’m working on them, but they are at retirement age and want to buy a motor home and travel.

The issue is that carpet is not really a suitable material for outside of a shower, so the solution to the problem is really yours not the tenants....as in I would be hard pressed to expect that a tenant should be subject to an opinion of what is a suitable sized floor mat for outside the shower....There is a bit of a subjectivity to the discoloration i.e. if they walked inside with motor oil on their shoes, obviously that is not a 'normal' level of soiling.

Otherwise I would look to Yonkers, Mount Vernon, New Rochelle or Jersey city area for house hacking, if you're primary job is in Manhattan.

You have a few hurdles to mount first. 

In addition to the items you mentioned, I have the HVAC serviced every spring including cleaning the condenser coils, lubricating the motors as needed and checking the refrigerant levels.

I feel this experience mounts the pressure to take care of her rent and shows her we are not push overs.