My back yard is at a slight downward slope. Is there a way I can even it out to have outdoor furniture sit better on a flatter surface?
Sounds like you need some excavating work down. I'd ask a local contractor/excavator about a project like this.
Well you do want a bit of a slope for rain an / or snow run off so that it flows away from any structure to avoid foundation issues and water intrusion. If it is more than that you can regrade the yard and bring in fill dirt.