Yes, you don't need a visa to Hk, for you will just stay there for 10 days.
For your trip to Shenzhen, you do need a single entry visa, not a 1-day visa. the enter before(period of validity) is diferent from the duration after enter. The single-entry visa will expire after you enter Shenzhen, but you can stay there for 30 days which is the duration of the visa. Hope I have cleared myself.

Thanks Mr Bruce. Can I ask where I can get the single entry visa to Shenzhen? I'm currently staying in Singapore. Do you know how much the single entry visa to Shenzhen costs?

With the single entry visa, i'll be able to enter Shenzhen then go back to Hong Kong on same day with no problems right?

Thanks so much for your help.

When you apply for the visa at the Chinese embassy in Singapore, the fee for single-entry visa is 50SGD. There is kind of Special Economic Zone Visa (use the visa can access only Shenzhen, Xiamen) available at the Luohu entry port of Shenzhen.

Hi Mr Frank,

Many thanks for your help. I read from websites that there is the visa available at the port but also read the due to H1N1 it is not as easily available. Can you confirm if I can get the visa immediately just before i enter Shenzhen? Thanks so much!

Yes, you can get a visa from the Chinese embassy in Singapore in advance. You can get the visa the next day after you submit your application if you use the rush service, but you have to pay some additional fee for it.