about visa for HongKong and main land China

Hi, don't worry, if you hold US passport, you are not required visa to Hong Kong for 90 days. It is ok to enter HK the both way to and back.

As Cecil answered, you don't need the visa for HK, but you do need a visa for China. Since it sounds like you aren't really interested in HK (which is a beautiful city and worth taking the time to see...) you might look into the ferry service from the HK airport to Shenzhen. You don't go through customs/immigration in HK, but do it in Shenzhen. You will save a lot of hassle that way,. China just announced changes in visa policy so you should check the consulate site to make sure you have all of the proper papers before you go to the consulate. You may need an invitation letter from your tour agency or the person you are visiting in Shenzhen.