I'm here in shanghai and planning to travel to Nanjing for a short tour on some tourist spots.

Yes, you can have a round-trip in one day.
There are high-speed trains between these two cities.
1. From Shanghai to NJ, the schedules are from 06:25 to 21:36 in Hongqiao Railway Station. The duration is about 2 hours. The ticket fare of first class seat is about CNY220 and second class seat is about CNY140.

2. From NJ South Railway Station to Shanghai, the trains are from 05:54 to 22:14. The ticket fare of first class seat is CNY230 and second class seat is CNY135.