You never mentioned which city you going,(GuangZhou city itself?"GuangDong" is a province, not a city.) so i cant be very particular, below is for overall GuangDong area.
SHOPPING(that applies to whole china):
You go there at the wrong time because most of the shops are selling winter wear already.
Department store that sells branded things, just convert to SGD see cheap anot, most department store cannot bargarin.
For small shops, MUST bargain.
SAFETY:
GuangDong area is famous for crimes like pick pocket, snatch and run, and some even more scary method of robbing/stealing you.
If you wear gold chain in singapore, leave it in singapore, dun wear it there.
Be alert, make sure no one touches you, bump in to you, if they do, check your wallet.
When something strange happen, like someone spill water to you and help you wipe your clothes, dun put your attention on your clothes, make sure no one pull your wallet from behind when the person infront wipe your clothes.
Try not to use those handphone pouch that attached to your belt. That's very easy to be snatched.
Amount of money to exchange really depends on how you want to spend.
But dun bring to much amount of money out, the biggest RMB note is 100, if you keep like 3000RMB in your wallet, it's gonna be thick, and when you pay money, ppl see your wallet so thick...........
Dun keep ALL your cash in one place for safety, so that if wallet got stolen, you can still take out 50RMB from another pocket to take a cab.
If you happen to take Bus(which i advice you not to), be very alert esp when it's crowded. Actaully no matter where, when it's crowded, beware.
Do not trust strangers, do not pity something that you see. ppl who con others, they make use of your greed or sympathy.
If you carry a bag, keep it slinged on you all the time in public, with the bad infront of you, not slinged behind.
WEATHER:
Not very cold if compared to norther area of china. check from
http://www.t7online.com/China.htm.
LANGUAGE:
Most ppl speaks mandarin, hardly any ppl can understand english.
Even you speak cantonese, they know you are not local, cos singaporean cantonese is very different in terms of the words used.
ENTERTAINMENT:
Massage is cheap, real cheap, i mean the decent type of massage, ask a local to bring you to one if you interested, or else you may end up in wrong type of massage center.
Some hair saloon can do body massage too(also decent one).
I dont know how old are you, if you want to go to "that kind of places", make sure you ask someone local to bring you there(Not local taxi driver or wat, i mean someone you know). If you adventure alone...... you may end up getting charged very expensive yet u must pay, being robbed, being caught by police(real police but it's a set up), or really caught by real police doing a raid. Recently quite alot of raid in Dong Guan city of Guang Dong province.
Most billiard center got young girls pick up ball for you, count score for you, replace the set of balls when you finish a game. Some can even play billiard with you.
Arcade and LAN shop, i dun see a reason why you should go there. Keep you belongings in your pockets/bags if you go there.