I'm going to Hongkong and plan to visit china as well just for a day. which part of china is near?

Shenzhen is near. You may take East Rail Line to Lok Ma Chau (Futian Port) or Lo Wu Port, and cross the border to Shenzhen. The trip takes about 1 hour. Afterwards, you can take a bus or metro lines to tour around. You can buy the tickets on the spot. By the way, you may go to see the following places: Window of the World, OCT East, Splendid China, etc.