For food I'd say go to Tsim Sha Tsui (TST), as it has a great choice of restaurants, much cheaper than Central has. 3 course Set Lunches in pubs and mid-range eateries there are only 8 to 10 USD during weekdays. Good bars there as well.
Go to Yau Mau Tai and Mongkok at night for shopping - Temple Street Market, for example
Try Sham Shui Po too, it has loads of copy and legit computer stuff in the Golden Shopping Centre, just outside the MTR station. Drinking and eating there are as cheap as chips, complete with a sleazy ambiance as it is a bit of a red light district.
If you need a girl, check the Miss148 or 161sex websites - they have the pics, addresses and prices of hundreds of hookers all over HK. For example, a pretty young Thai hooker in an apartment in Sham Shui Po will cost about 35 - 40 USD all-in for a half hour of sex. Chinese girls are often slightly more expensive.
While in HK get an Octopus Card at any MTR station, you can use it to pay on all the public transport, 7-11s, McDonald's, in fact most shops, saving a lot of hastle.
For sightseeing I'd just go on the MTR, head to the New Territories and get of at random stations. Look around, it's a whole other world. Or, head to the outlying islands such a Lamma or Peng Chau - you can get a ferry from the outlying islands ferry pier in Central.