Results ahead of time and overdelivering

I own a house cleaning business and I was wondering what I could do to provide results ahead of time. One of my ideas is 1 free bedroom/den cleaning. I do estimates and discuss the cleaning options with the homeowner, then book the job on the spot. I don’t know how I could just go in and clean 1 room for free. And not complete the rest the cleaning. This seems unproductive. I’ve been in biz for 25+ years and always overdeliver whether that be in the cleaning itself or customer service, what other ways can I overdeliver?