I can add value to RE by adding a bathroom, renovating the kitchen, ADU, etc.

I don't think there's any one platform that gets gold stars across the board. 

(Of course, there is a factor that higher rating will increase my booking but that is not just coming from adding one or two amenities) Besides item 5 and 6, I am not sure how much other items will attract more guests to my business.

SMS, Direct mail, facebook ads? 

A monthly fee (typically around $5-20/mo) is added on to the payment.

I recently added new properties in a new city and posted a craigslist ad but got no response.

Adding 1-2 doors this year would be awesome. 

I personally haven't sent postcards, I've had good luck with PPC and Video ads. 

Here is a list of why adding ADUs in my CA market is typically a poor RE investment:1) The value added by the ADU addition is often significantly less than the cost of adding the ADU.

After acquisition, we furnished it, added a couple decks for a hot tub and solo stove fire pit, as well as a ton of string lights and were off to the races!