Well, great idea; but unusual in Hong Kong – of course partly as so few people have gardens, also as not something I've really heard of people doing.
For plants, you might get some ideas from places like Kadoorie Farm, Fung Yuen Butterfly Reserve (near Tai Po Market).
Consider some species that are good for wildlife like butterflies, some birds. Coral tree is among these: not native, but attractive flowers around this time of year that can be great draws for Fork-tailed Sunbirds, which are somewhat akin to hummingbirds (which aren't present in Old World). Prime examples include trees in front of management centre of Tai Po Kau Forest Reserve, which attract birds like white-eyes, sunbirds, and in turn draw photographers.
Bird food can be varied, mainly winter and especially in chill spells. Might include leftover food, like rice, pasta, bread, fruit. I don't know of people feeding seeds; could prove interesting. Some photographers use mealworms to attract some shy insectivores (not in gardens, but in and near woods I think).
Bat boxes could be worth a try. For bird boxes, could look at UK designs, for birds such as Great Tit (which occurs here, tho somewhat smaller race/species).
Log piles: I doubt necessary. Might draw some less wanted wildllife, like snakes, giant centipedes.
Small pond could be good, inc for frogs (tho these can be very noisy on spring nights!).