Have you been to the other islands around HKG? You could take a bus ride and sit on the second floor and watch the scenery, go to the fishing villages on Lantau Island, or take a ferry to Lamma island.......no need to go to nearby countries.
There are so many hills in HKG where you could hike, there are trails everywhere.....
People tend to think about HKG as a shopping place and only of HK island, and kowloon, but there is so much more!
Last trip, I went to one of the offshore islands, Lamma island. Pretty interesting island. Sort of getting a "rural" break away from the mad rush and the mobs of people in the city. And for us Singaporean that is so used to have so much green around us. It is really refreshing to see green again.