Let's make a public park, but put it in the SKY.
Salesforce Park sits on the roof of the transit center and runs 4 blocks long. There's security 24/7 and the park is immaculate.
The free gondola was sadly out of order, but the kids loved the bus fountain, which shoots up water when a bus passes underneath.
Here's a link to my story for Seattle's Child: family-friendly adventures in San Francisco.