OMG hot and humid, its terrible but worth it.
Where are you staying HK side or Kowloon side. I know there are a few AT guys that live in HK that will definitely give you a better list than I.
When I went last summer, I went to Mongkok for electronics/computer stuff. I also went to Victorias peak for the view, it will be awesome at night.
Central, because its bascially downtown and all the shopping areas are there like times square.
Tsim Tsa Tsui in Mongkok is also nice, walk down Nathan road.
Lantau island to see the Buddha statue, kind of a long bus ride. The hike up the stairs in 100 degree weather may kill you.
Definitely take the star ferry at night from kowloon back to HK, the view is awesome again.
Thats off the top of my head, I'm sure there is tons more.