What options are there in terms of prepaid Internet services? We will only be there for five days or so.
You'll want to decide whether you need to make calls, send SMS, or if you're just after it for data.
Then take a look at the Prepaid Sim Card Wiki entry for Hong Kong.
Depending on what phone you have, you have a few options there, and it'll depend on how much data you plan on using in those five days, of course, as to which one suits you the best, but you'll be able to choose between 3 KHK, One2Free NextG and PEOPLE's, each with their pros and cons.
We found that 3 gave the best coverage and price when we were in Hong Kong last month.
It's highly recommended you go to one of the numerous "Computer Centers" (eg Wan Chai Computer Center) to buy it. They will be more inclined to set it up for you and if it doesn't work, they will usually refund you.
Top-ups are easy to get from a 7-11.
Yes, you can get a SIM card from a 7-11 but you are out of luck if it doesn't work and you don't speak Chinese.