Mongkok is a bustling district where Hong Kong's youthfulness, energy and creativity collides with old school culture, buildings and markets. It is a thriving district that is packed with crowds of all ages going about their respective lives and leisure interests.
There are numerous street markets scattered around Mongkok, one that I like is the Nelson Street Wet Market found on the west side of Nathan Road and Langham Place shopping mall and situated between Tong Mi Road (west) and Reclamation Street (east).
Nelson Street is lined with vegetable, fruit, meat, seafood, sundries dealers. You'll find loads of delicious fresh food as well as peculiar finds including fresh tofu blocks, thousand year eggs, Chinese medicine packs, frogs and other things you will rarely if ever see in western wet (food) markets.
The market is under large red umbrellas offering shade during midday when the sun is out. Still better to go from mid to late morning (9.30am) when stalls are stocked up and at their best for pictures. The trade-off is that the street can be packed with shoppers at this time of day as well.
In the end, you can actually traditional wet markets all around Hong Kong. If you are in Mongkok and interested in seeing one, this is a convenient and fun one to visit. Otherwise, it is rather easy to find good options elsewhere around the city as well.