Is Shanghai enough, or should you add Suzhou or Hangzhou?
Suzhou and Hangzhou can add gardens, canals and lake scenery, but they should not be automatic. Add one if it changes the rhythm; skip both if Shanghai is already short on breathing room.
Answer
The rail map makes Suzhou and Hangzhou look easy, and they can be. The question is whether they add the kind of day you actually want. Suzhou is usually the better choice for gardens and canal-city texture; Hangzhou leans toward lake scenery and a broader, slower day. Both can become tiring if they are squeezed into a short Shanghai stay already full of neighbourhoods, food and museums. A good day trip should change the mood of the route, not simply prove you knew there was a fast train.