I lived in Hong Kong for almost 5 months, and I spent about a month in Thailand so:
Hong Kong is really cool. So much going on and such a juxtaposition in itself. You can go for a meal worth easily 50 - 100 USD in Soho, or find some small little place in Kowloon where you can have wicked food for under 5. Same goes for clothes, drinks, everything.
Same goes for HK's demography and geography. It's a common misconception that HK is all city. It's not. There are some virtually dead beaches, and tiny islands with fishing villages. So as far as tourist things go I'd suggest. Big Buddha and Tai O (pink dolphins, village) on Lantau Island. Lan Kwai Fong and Wan Chai for drinking and partying on HK Island and Big Wave or Sharks Bay for beaches on the south side of the island. Beaches might be getting too cold now. Check out the peak for a wicked view. Some temple as well that I can't remember the name of that is pretty cool. Ride the tram. Happy Valley horse races.
Kowloon side has the Promenade and museums. 10000 Buddhas monastery. Check out some of the malls to be like a true HKer. Marvel at the skyline. Mong Kok and its markets. Nathan Rd.
Just generally explore the place.
As far as Thailand goes. Go there for partying. If you are going there for culture, then... well I'd personally suggest, don't. Like the guy above said. The islands are super fun. Full Moon party! If you don't already have a diving license, get it at Ko Tao for cheap. Lots of good food too. Head to northern Thailand for some Treks, but do some research for authentic ones, because they aren't all such. Bangkok is crazy. I don't think a city can have a more suitable sounding name. Lots of tourists. Too many. That goes for most of Thailand. Try get somewhere off the beaten path... or:
My personal suggestion, not to slander Thailand and downplay your trip, because its wicked, head to Laos or Vietnam. Vietnam = less tourists, more culture (because of less tourists,) beaches, diving, surfing, cheaper, more diverse (landscape wise). Laos = super chill and happy. I mean, there is a restaurant in Sei Phan Don that sells everything on its menu as either normal or "happy." Happy!