Fish tanks, computer equipment, lighting, heating pads, portable air conditioner, electric blankets, power tools, just on and on. 

But, as unlikely as fires are, I couldn't live with myself if a tenant died because they couldn't escape out the window:https://www.amazon.com/BLACK-DECKER-BPACT08WT-Portable-Conditioner/dp/B01DLPUWL2/ref=sr_1_1?

As far as a tenant installing an air conditioner, I'm not worried about that too much as it's not a complicated process.

For cooling, we rely on seven window unit air conditioners and free-standing window units. 

Add a few window air conditioners you don’t have enough power.  

Me: "Do you mean your air conditioner isn't working?" 

We have one of these heat pump water heaters, it is like an air conditioner in the space you put it in and ours is a tank heater.

Shampoo and Conditioner seem to be pretty common.

If your hoping for repeat guests (where they pay full rate and you don’t have to share it with Airbnb or VRBO), then provide hotel size shampoo, conditioner, body wash, small toothbrush and small tube of tooth paste.  

They did "2 things you should never be without..." and when the person visited the website it talked about you should never be without an Air Conditioner...