>>>>Avoid places like Mong Kok markets, Big Buddha, Sai Kung, Peak Tram, Ocean Park on weekends. <<<<
Actually, we often go to Sai Kung for the weekends because it is so much less crowded than the main areas of the city. The Promenade area does take its fair share of visitors but its nothing like walking along the TST Promenade or Ave. of Stars. The rest of Sai Kung is quite relaxed, nice places for alfresco dining and places to walk and enjoy yourself without rubbing shoulders with people the entire time.
Stanley is another weekend area we like, much less crowded than the main areas of Hong Kong on weekends.
Suggestion above for Nan Lian Garden and Chi LIn Gardens is a good one. Many of the city museums as well are never crowded and worth considering. Consider the Hong Kong Heritage Museum and the Hong Kong Coastal Defence Museum, we've never seen crowds at these very good museums.