We just put in a swingset, and we are about to put in an arbor/trellis for shade and a playhouse. Note that this is a lower-end affordable-housing type park. We also built in picnic tables (the benches are connected to the legs and it is all embedded in the ground). The cost was minimal. The benefit to the community cannot be measured but making it a nicer place to live will pay off in the long run in decreased turnover, lower cap rate, and general goodwill.
I would like to point out that as a landlord, you have a social obligation to do more than squeeze every possible dollar out of the tenants into your own pocket.
You should discuss with your insurance agent but the additional liability cost is minimal.