If you are travelling for such a short time, it may be best to look out for wifi hotspots. Starbucks offers 30mins free wifi with purchase. Most hotels and even hostels in Chungking Mansions offers complimentary wifi when you are a guest.
If you have your heart set on using the 3G network on your iPad, 3 offers an iPad Starter Kit for 3G and wifi for HK$198 which is a rechargeable SIM. There is a monthly usage charge of HK$18 that will eventually eat up all the credit if you do not use it. You can recharge at any 7-11 store and they're everywhere!
Check out: http://www.three.com.hk/website/appmanager/three/home?_nfpb=true&_pageLabel=P200470391219567710594&lang=eng&pageid=61I001
Another option is if you have an iPhone 4 or above, to get a prepaid SIM from any 7-11 store and use the personal hotspot option with your iPad.
Hope that helps. Enjoy HK! :) over 6 years ago