Trying to build a sports park, I have a question about releasing the city from liability.

The city is willing to help us build a public sports park, but we have to somehow ensure that the city will not be held liable for anyone who gets injured. Some ideas from the park and recs guy was issuing certification cards to people who signed a waiver and passed a physical exam, and/or providing insurance.

Will these cards be enough to release the city from liability? Also, how do we provide insurance coverage for anyone visiting the park if it's a public park?

Thanks in advance!