An overnight trip solves the problem a single day in Suzhou creates: there is more worth seeing than a day allows, and cramming it in on the day after a marathon is unnecessary when a second, gentler day is available.
High-speed trains from Shanghai Hongqiao reach Suzhou in about 30 minutes, running frequently through the day. Tongli, a smaller and quieter water town, sits a short taxi or bus ride south-east of Suzhou proper, and is best treated as the second day's slower pace-setter once the legs have had a night to recover.
Day one: Suzhou gardens and old town
Spend the first afternoon and evening in Suzhou itself. The Humble Administrator's Garden or the Lingering Garden make a manageable single stop, and the Pingjiang Road canal street is flat and well suited to an early-evening stroll and dinner. Check into a hotel near the old town rather than the modern district, so the walk from dinner back to bed stays short.
Day two: Tongli water town
Tongli, smaller and less crowded than Suzhou's old town, is built around a network of canals and stone bridges best explored on foot at a slow pace, with boat rides available for anyone who wants to sit rather than walk. The town is compact enough to see thoroughly in a morning, making it a good second day for legs that are recovering but not yet fully restored.
Getting back
Return to Shanghai via a taxi or bus back to Suzhou, then a high-speed train from Suzhou station to Hongqiao. Trains run frequently until late evening, so there is no need to plan the exact return time in advance.