The weather will fine in Shanghai and its around cities. April should be one of the best time to visit this City. There are many attractive places in Shanghai, please choose some from the list at https://www.travelchinaguide.com/attraction/shanghai/

For further traveling, you could go to Suzhou City and Hangzhou City. Or, stay for one night in a water town, such as Xitang, Wuzhen or Tongli. The transportation from Shanghai to these places is very convenient.