Hi Jos,
Greeting from Pegy, Top China Travel
For Shenzhen short visiting recommendation, I think you can check about the comments on TripAdvisor
Here is the link to know some more about Things to do in Shenzhen
http://www.tripadvisor.com/Attractions-g297415-Activities-Shenzhen_Guangdong.html
From my opinion, a day visit to Happy Valley or OCT East Shenzhen will be joyful.While it is also depends on your interest. So you can firstly check about the comments on TripAdvisor and I will help you to clear more about it.
And if you plan to visit another city in China, it also should depend on your interest, such as
Beijing and Xian for history
Shanghai for modern China
Guilin for beautiful scenery
Guangzhou and Hong Kong for shopping
etc...
Feel free to contact with us in any moment by mail if you have any question on China tour service.
Have a nice day!
Best Regards
Peggy