The Narrabeen Lagoon Trail is an enjoyable 8.4 kilometre track around Narrabeen Lakes, suitable for walkers, joggers and cyclists. Its flat terrain makes it accessible for everyone, with a series of  boardwalks, bridges and tracks  winding around the shoreline. There is an abundance of wildlife and native bushland, with areas on the lake for kayaking, stand up paddle boarding  and sailing. On Tuesdays, model sailboat skippers hold a regatta at Jamieson Park.
 
You can begin from many locations: Berry Reserve on the eastern side of the lake; Billarong Reserve on Wakehurst Parkway;  Middle Creek Reserve,  near the Sydney Academy of Sport and Recreation on Wakehurst Parkway; or Jamieson Park on the south side. Picnic areas are provided along the trail along with toilet facilities.
 
It's a great adventure, but it can be little busy on weekends, so spending time there mid-week is generally more relaxing.
 
A great website for all the information you need is Best Sydney Walks.