Thanks for this report. (For others who read: the trail that avoids Yi O is good, with grand views over sw Lantau and Pearl River mouth – but indeed involves some climbing).
Without checking maps re lengths, some flat or fairly level hikes that occur to me:
From Tai O towards northeast, and round to Tung Chung. Undulates in places, with some flights of steps.
Around Shing Mun Reservoir: not so long, but a fine walk.
High Island Reservoir southern edge – level, and takes you to eastern dam, which is impressive place, w cliffs w columnar jointing in the area. But, then either retrace steps, or some ups n downs before walking down the road to north of the res.
Catchwaters in several places. One along southern slopes of main Lantau hills is good I'm told (inc for cycling).
From Hoi Hal westwards, to Sham Chung and then south thro Yung Shu O involves only a little uphill, which is pretty gentle.
A few circuits of Tung Ping Chau!
Hopefully some other folk can suggest more. Martin