After you get off the taxi at Wan Chai Gap, head on into the small playground, and have a crunch at the food kiosk before heading out. There's also a toilet there, and some tables for a quick lunch or an afternoon snack. After that find a road called Aberdeen Reservoir Road (Also the Aberdeen Tree Walk). That would take you straight through Aberdeen Country Park. There is also a sign marking the Aberdeen Country Park. 

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After walking along the road for about 10 mins, you should see a path with a signpost marking the Hong Kong Trail. Walk along the small trail, and you would soon find yourself in a forest. Crossing some streams, you would soon feel refreshed and relaxed.