Take the tram or bus up to Victoria Peak, it's worth the view.
The nightly laser show (not sure if they still do it) is kind of fun.
Going on the Star Ferry (from/to Tsim Sha Tsui) is fun.
We enjoyed going to the night markets and bartering with the vendors
My family enjoyed going to see the huge Buddha statue (Tian Tan Buddha)

Also, if you happen to be there for church, make sure to go to the English-speaking ward at the building on HK island. There are some other meetings at smaller buildings, so if you just pick one off of the church's website, it's likely to be a meeting for Filipino women who are working as domestic workers (housekeepers & nannies). I showed up to a random church building on my first Sunday, and it was me, four missionaries, and 100+ Filipino ladies.