Mong Kok is famous for shopping. In particular it's incredibly long street with 1000's of market stalls and shops. Here you can buy pretty much anything, and will easily spend hours wandering around, bartering and browsing.
Me walking through Mong Kok market
Pet fish for sale in Mong Kok
Mong Kok at night
At night Mong Kok is a lively place with all the shops and stalls open until late, lots of restaurants and lots of people, and a good fun atmosphere.