We installed HVAC + ductwork instead of keeping the window unit, swapped the 120 amp panel for a 200 amp and changed the old cast iron and copper plumbing to PEX and PVC.New roof, floors, cabinets, appliances, light fixtures, paint, etc, etc, etc What made you interested in investing in this type of deal?

Sometimes a little paint and the addition or swapping of accessories is all that is needed.

I would definitely swap out the oil if NG is available.

@Rohit Verma The lock swap out really needs the doors to be a better grade.

Again the mayonnaise is heavy here.

The husband comes outside to shoot the breeze and I ask about the house, if they are having any issues, he says no but wanted to ask me about swapping the shower/tub in the guest bathroom to one of those walk-in bathtubs...He said his wife has trouble lifting her legs into the shower as it is and that they would pay for the tub and install.

Even after we got the renters in, we've continued to have issues (plumbing lines swapped - under warranty; electrical in back of house didn't work - we never checked back there and didn't know there was an issue until renters moved in; bought used appliances and had several warranty issues and just had to buy a new dishwasher.).For my wife, who managed the project, it was a huge undertaking and now we're doing 2 more this summer.

We've swapped out the old 40 gallon hot water tank (for a house that sleeps 10!)

Granted, I have one local rental unit so it wouldn't be something I do at scale, but the thought was that perhaps they would remember the (cheap-ish) bottle of champagne before calling me on a weeknight for something simple like a light bulb swap. 

I wish to swap the key was more like the Smartkey from kwikset.