First off - welcome to the forum and i think this is a nice thing that you are proposing for your tenants.
I'd say definitely look into the insurance thing - or put up a sign that says 'area/ dogpark' to be used at your own risk. Make sure it's all legal.
A set of rules on the fence (needs to have a secure fence) is probably a good thing as well.
- a garbage can or two with a secure cover would be wise. maybe old chairs or milk crates (or better if u have) for the humans to sit on.
- Something to hold plastic bags so that people can pick up after their pet. (should be rule # 1 or 2 :0 )
if more ideas pop, I'll add them in.