Why don't you go peak at night for sight seeing at Hong Kong Night View. It's really wonderful.
If you stay at Mong Kok, you may go Women Street/ Fa Yuen Street/ Temple Street@ Night/ Longham Place. Go Women Street/ Fa Yuen Street/ Temple Street please remeber to have a bargain fun.
Try to ask 50%off.
Go Tsim Sha Tsui if you want to shopping for more trendy and fashion stuff. Will spend more expensive than Mong Kok.
Go Ocean Park for a day, will give a lot of fun.
Must try our egg tart and milk tea in our cantonese resturant.
Must go "Ki Tsui Cake Shop" as i mentioned before. Try out their "Siu Ben"
Must go "Lang Kwai Fong@ Central" if you like night club.