I give my tenants choices all the time, in fact at lease anniversary time they can choose 1, 2, 3 year lease, new window air conditioners, ceiling fans, paint….. but I show them their rent will increase based on the improvement option they choose.AC is a big one: so I spend $150.00 on a window shaker, divide that by 12, add a service charge for the install… and viola’ we have a new number to add to the rent.

As soon as I get a notice, I let tenant know the apartment will be listed for showing, I set appointments and give tenant 24 hour notice that I will be showing the apartment within a 2 hour window, the sooner shown the sooner rented..I don't do a open house, I do only by appointment.

With tinted windows you lock it at night and all your tools are safe and ready to go the next day.Affordable and practical.

I assume  you're referring to central AC, and not just window air conditioners?  

Here's where I suggest throwing the Phoenix area out the window.

My tenants have paid to upgrade all the apartments and buildings that I have ever purchased - new windows (every single one of my units has them), new floors, doors, fixtures, counter tops, appliances, paint, etc., etc.

I was addicted.2) We decided to make our first procurement a rental for his son while he was going to college (slow Flip is what we called it)  After we found this house we were not going to be able to get it ready in time for school  and that strategy was out the window.3) Got on bigger pockets market place and found a house in Indianapolis and went to look at it.  

To keep the city from tearing the house down they had to put on a new roof and new windows.

Saw one where the windows changed 5 times.  

I'd clean it off use a solution that has mild bleach in it.. check the crawl space below and exterior walls, or windows, or any roof leaks that are draining toward that area..