Obviously from HK, China is on your doorstep - but you will need a visa so not an option for weekends. Plenty of people tour in SW China, I'm en-route (waiting for Visa) now.
Lao is wonderful - new roads are being built and touring options are opening up all he time. Very cheap and friendly place.
Vietnam is also great - there is a good 10 - 15 day tour from Sappa back to Hanoi via Dien Bien Phu - big hills but wonderful scenery. Lots of people go from Hanoi to Saigon pr VV depending on time of year (wind direction). There is also a new N>S highway through the mountains that looks like fun (Ho Chi Minh Highway).
Unless you like off road stuff the cycling in Cambodia is less spectacular - a bit flat, but plenty of culture at Angkor Wat and good beaches in the south.
As said previously, Thailand is excellent, particularly in the north (easy train access from Bangkok). You can fly very cheaply (30 quid) from Macau to Bkk, no visa reqd. Many low cost airlines take bikes eg Air Asia for 7 quid.
I agree, forget your campimg gear for SE Asia - the hotels are usually more like a fiver or less. The temperatures can be pretty cold in S China, N Vietnam, etc in the winter -so depending on what you are doing you may need some warm clothing.
The Mr Pumpy website is fun, but very out of date now. I suggest you look at the Lonely Planet one
http://www.lonelyplanet.com/thorntree/f ... forumID=32 - plenty of SEA info there.
crazyguyonabike.com also has plenty of tour write ups.