One of the more adventurous trails in Sai Kung has been going via Pak Tam Chung Nature trail until the reservoir, and there continuing on Sai Kung Sai Wan road until Sai Wan beach (also Maclehose 2 trail takes you but from opposite direction).
The first part of the trail is on Pak Tam Chung nature trail and is flat and follows the sea.Then, you have 2 options, one of which is Sheung Yiu Country Trail which takes you through a campsite and eventually to the reservoir.
You can end your hike there, or if you want to hike longer as we did, take the road to the left, until you come to Sai Kung Sai Wan road and turn right. At the end of this road, you can take then a scenic trail to Sai Wan beach through the hill. The beach is an excellent place for some relaxing, simple food and swimming before taking a boat back to Sai Kung.