One of Shanghai's nicest parks, and also the location of the Strawberry and JZ music festivals. Easy to access by Metro Line 8, China Art Museum station (Exit 4). It's about a ten minute walk. A cab from downtown will probably cost around 50-70rmb, and take about 25-30 minutes, depending on traffic.
Important: The entrance to the 2015 JZ Festival is now closer to Shibo Da Dao, near Shangnan Lu (世博大道1200号, 近上南路). So, right by the Benz Arena, and closer to the metro station.