Normally, one or two days are enough to cover the main attractions. The must visit places are the Bund, Nanjing Road, Yuyuan Garden (and nearby Yuyuan market if interested), Huangpu River. Besides, you may also visit Jade Buddha Temple and Xin Tian Di. If you still have time, paying a visit to water town around or Suzhou or Hangzhou by high speed trains are all good options. April is one of the best months for traveling. The temperature is comfortable.