I have 7 hours transit in Nanjing Airport to Singapore.

You can go to see the interesting places at the city proper, like Confucious Temple, Presidential Palace, and Qinhuai River. From the airport, take metro S1 line to South Railway Station (about 35 minutes), and switch to metro line 3 to Confucious Temple Station. After touring around, you can take a taxi back to NKG for about CNY 110. The trip takes about 45 minutes. Remember to return to NKG at least two hours before departure! ;)