Jonny recommends
Congee - is like porrige and made with flaked rice. It is salty and finished with coriander and finely sliced spring onions. The meat can be chicken, frog, beef pigs intestines, anything really. I had chicken congee. It cost 1.50 only so a good dish for budgetters. Typically the meat is hacked up on the bone so you'd better become addept woth the chopsticks.
Ginseng tea - visually the weirdest thing I have ever taken from a cup. Long on the palate with a medicinal, gingery, earthy, slightly tangy taste. If you have seen "Alien," close your eyes when drinking.
Chinese noodles - go to "Traditional Chinese Noodle shop" on Ashley road, Kowloon for a variety of home made noodles. I ate there several times. Most memorably I ate large wontons of minced pork, lemongrass and chilli in a rich, meaty soup balanced by lemon. Amazingly good value.
Choi Sum - vibrant green vegetable served everywhere. Very tasting with oyster sauce.
Street snacks - most are deep fried but there are a myriad of savoury treats on sticks. Typically seafood pieces including squid and octopus, tripe and smoked duck. Coconut balls send heavenly wafts down the street which alleviates the vile sewage smell nicely.
Egg tarts - I wouldn't bother. Our UK ones with nutmeg are lighter and have more flavour, the French generally do the best pastry in the world. For the ultimate custard tart I would actually have to go to Lisbon (my birth place) and eat Pastel de Nata. Phenominal and unique elasticcy pastry and obligatory burnt top.
Dim Sum - This is the place to eat the many steamed and fried parcels offered. Sweet or savoury, they are varied and delightful to share.