One of my favorite easily reachable hikes in Kowloon is the combination of Little Hawaii and Wilson Trail 3 which is a medium level hike and roughly 13km long.
Little Hawaii trail starts in Po Lam and is a relatively relaxed trail with a lot of green and not too much climbing.
If you don’t feel like hiking very long, you can always just do the Little Hawaii part, but if you’re like me and prefer hikes of 10km or more with some climbing, then continuing the Wilson Trail 3 in opposite direction is a good option (they connect via Ma Yau Tong road). Wilson Trail 3 includes much more climbing (some of it on steps) but also offers amazing 360 views over Hong Kong from the Black Hill and the Devil’s Peak. The trail ends (or actually starts) at a small village of Yau Tong where you can either hop on a ferry to cross to Hong Kong Island or take the MTR.