Suzhou is 90 minutes by high-speed train and offers classical gardens, silk markets, and canals in sharp contrast to Nanjing's historical monuments. It is the natural day-trip companion from Nanjing.
Yangzhou, across the Yangtze to the north, was one of the wealthiest cities in Tang-dynasty China and preserves several excellent classical gardens and a well-maintained old town quarter. About 1.5 hours by high-speed rail.
Zhenjiang, 40 minutes east by train, is a smaller city on the Yangtze with three famous hills, an old British Concession area, and Jinshan Temple dramatically situated on a rock in the river.
Jurong to the southeast has Mao Shan, a mountain sacred to Taoism and a half-day excursion for those interested in Chinese religious sites.