Huaihai Park, like many other parks in the City, used to be a Cemetery.
In 1863, the foregin settlement purchased a site that faced Avenue Joffre, today's Huaihai Road, which was within the Chinese territory at the time and connected with the International Settlement via Cemetery Road. As people had to pass by the Baxian Bridge (literally the Eight Immortals Bridge which ran over teh river immediately to the south of the park) to get to the cemetery, the cemetery was named the Baxian Bridge Cemetery. It covered 3.2 hectares and was nearly filled by the 1940s. It was moved in the early 50's to create the park we see today.
More information can be found here..
http://www.jewsofchina.org/shanghai%E2%80%99s-lost-foreigner-cemeteries
https://www.globaltimes.cn/content/831024.shtml
The trees at the north of the park are still the original trees.
To find the Geocache, simply count the number of stand alone exercise equipement in the park. This is a number (X)
The Geocache is located at
31 N 13.(502-X)
121E 28.(387+X)
Have fun, and perhaps check out the fire Station and training gym at the north of the park.