Perhaps this thread would be better suited to the HKG forum, where there are threads about what to do on HKG layovers. For the HK experience, the two things that come to mind are walking the Tsim Sha Tsui promenade for the view of Victoria Harbour or going up to the Peak for the city view from above and behind.

To TST: Airport Express to Kowloon then taxi to the Intercontinental Hotel. Alternatively, Airport Express to Cheung Wan then change to MTR to Tsim Sha Tsui.

To Peak: Airport Express to HK Island terminus, then taxi to Peak Tram and tram to the top, or just take the taxi to the top.

Peak would take significantly more time than TST.