I know some one-day tour to the water towns around Suzhou (around 150RMB), supplied by the Shanghai Tourist Transport Center. But, seems that few one-day tour in downtown area. Personally, I think there is no need to participate in a group since the main scenic spots in Suzhu are very close to each other and the city-buses could take to from Suzhou Railway Station to these places easily. What's more, the trains running between Shanghai and Suzhou are frequent~