Very popular on Public Holiday's and at the weekend, especially on a Sunday with locals and foreigners alike. Walks and parks are generally in the built up areas and are within the reach of all capabilities, both young and not so. All the country parks and beaches have a barbeque area. Park n' Shop and Welcome supermarkets sell all the necessary items you need.
The numerous Country Parks around Hong Kong, New Territories and Lantau are considerably bigger and more challenging. Some are quite hard work, so be prepared, make the necessary preparations like good shoes, water, high energy foods, phone, map, water proof or sun block depending on the time of year. They have markers with grid references at strategic locations should you get into any difficulties. Unfortunately every year there are a number of accidents and there have even been fatalities. So be prepared and don't go on your own and always tell someone where your group is going.
The trails on the islands such as Lamma and Chin Chau are a good compromise between the two previous choices, more challenging than the manicured paved areas of the parks, but not the full hiking experience of the Country Parks.
As an alternative there is also the Hong Kong Wetland Park, the marine parks and reserves.
Finally, it's best to check the weather before before you leave, it can change surprisingly quickly, so be prepared.