This sounds like the better approach. Create a trust that runs a private park open to the public. This prevents the city from owning the land. The trust can also work out a deal with the city for tax benefits for running the park. The trust can also be set up so that a family member is always given an overriding voice while allowing the city to submit plans for proposed use, upgrades, permitting, etc.

Basically you need to pay a lawyer to set up a trust which requires trustees if you care or donate to an institution with their own lawyers who you trust with a presumably long institutional timeline.