Think adding another bedroom (3 bedrooms tend to be a great sweet spot for rentals), or doing some small cosmetic updates (think paint, hardware, fixtures, floors... anything that increases the appeal of the home without costing you a ton of capital).

New paint and hardware goes a long way.

The metadata on the picture will indicate the time and date I stuck it on.Then I'd go to a hardware store and buy a deadbolt.At the agreed-on time, I would come back and start beating on the door as loudly as possible, shouting that I am the landlord making an inspection visit.

I managed it myself and just had to clean the inside up a bit, add some cabinet hardware, and replace some older lights and this was a very easily rented condo.

What I would have to do is a basic interior remodel as units become vacant which will cost around $5,000 each: paint, flooring, light fixtures, faucets, revamp kitchen cabinets, door knobs and hardware, etc.

I search for Pottery barn and restoration hardware and pier one and I have done very very well with furniture that looks brand new for a fraction of the cost.

I'd consider selling the hardware at a loss (or even give it out for free), because the revenue model would be based on a per-door low subscription fee.

How much of a difference in your nightly rate/annual revenue is it going to make to have a rehabbed kitchen vs. maybe just changing the hardware?  

I'd paint the cupboards white and keep the existing hardware.

If the tenant only wants to prevent others from entering to feel safer, this is best done with hardware (robust locks and doors)