I was posted to Hong Kong for work for about half a year not too long ago. I brought along a combo of D90 with 35mm f1.8 and the kit lens 18-105mm f/3.5-5.6. I also brought a tripod.
I brought both lens because they were the only lens I had in my possession. I was using them to take the following images of :
1) my 2-month old daughter who was growing and changing with every passing day while she made the trip with my wife and me to Hong Kong.
2) food and gloriously good food (from 茶餐厅 to Yum Chas to Michelin-starred restaurants)
3) as I was on a business trip, I was taking photos at nights on Mondays-Fridays and day/night shots on weekends.
4) Landscape shots were taken at places such as The Peak, Lantau Island and Lamma, Big Buddha, Stanley, North Point, Mong Kok, Hong Kong Island from Sheung Wan to Tai Koo and Kowloon to Lai Chi Kok and even Yuen Long and Ma On Shan!
5) Theme parks that I had visited included Disneyland and Ocean Park.
6) Weekend getaways to Macau as well.
During my time there, I must say that I had 80% of my shots using the 35mm. Since I take mostly images of food and my family, the 35mm f/1.8 was able to capture suitably well in low lights. The only times when the 18mm was needed were for:
i) the nightly laser shows from Tsim Sha Tsui overlooking Hong Kong Island and vice versa. (even the 18mm is not able to capture the entire Hong Kong Island)
ii) the view from the Peak (the 18mm was also not able to capture the entire view in 1 shot)
iii) views from Lamma Island
iv) shows at theme parks where I ensured that I'm seated at the "No splash zone" so as to protect my camera equipment.
v) The Ruins of St. Paul in Macau
Since TS does not have any intention to switch to FF, a 35mm lens is effective about 50mm already so it does present a tight angle to shoot with. Fortunately for me, the 35mm focuses to about 1 foot so I never really have to stand up to take a picture of a food in a busy and cramped 茶餐厅 or Yum Cha place around Hong Kong)
At the Avenue of Stars, I was taking the statue of Bruce Lee in low light (The sun sets around 5:30-6pm in Hong Kong). Using a tripod, I was able to set a longer exposure with the 35mm fully open compared to the 18-105 kit lens at f/3.5. With the f/3.5, my subjects were moving like they were practicing 無影腳 (shadowless kick)!
Changing lens is quite challenging in Hong Kong due to bad streetside air quality (very high PSI readings) and strong winds. It is advisable to change your lens indoors before you go outdoors. At places like The Peak and Avenue of Stars, I was running indoors to change my lens.
I hope you will enjoy your trip to Hong Kong. Regardless of the lens you bring, it is more important to enjoy the trip than to worry about the adequecies of the gear you bring. I did not bring my tripod every day as well and ocasionally relied on my wife's PnS camera or the camera phone for those times when I went jogging at Happy Valley Racecourse or to Stanley and to some cosy cafes with free wireless access point.
i brought my 35, 17-40 and 135 and end up using 35 + 17-40 mainly and most of the time i am using 35 haha. My 135 I only used once for less then 5 mins.