To answer your actual question:
* groundsheet (even if your tent has one built in)
* sleeping mattress (have you ever TRIED sleeping on the ground before? It's hard. And very uncomfortable
* sleeping bag/bags. Lightweight in this weather, but even so, in the middle of the night you might get cold
* rubbish bag - leave nothing behind
* breakfast
* ideally also - small camping stove to heat up water and make tea in the morning/evening; cook food on - no, you can't cook food on a fire. It's almost impossible.
* rucksac or equivalent to put it all in
Also, make sure you have, or have access to, water. You'd be amazed how bad some camping spots in HK can be in this respect.
But I too endorse the comments of the others that Pui O is possibly not the best place to camp. Far to busy and you miss the actual benefits of camping, which are the natural peace and quiet, seclusions and the ability to watch stars at night and wake up to a fresh new day with dew on the grass and nobody else around.... NONE of that are you going to get in Pui O. But you will get sand in places you really never wanted to !
EDIT - And don't forget the toilet paper either!