If there is one not to miss event in Hong Kong, it's watching the city at night. Kowloon is fairly impressive at night, but Hong Kong Island is mind blowing. The number of tall modern buildings and bright lights is a photographers dream. In fact the biggest problems you're going to have is getting everything in on one shot. I recommend making sure you have a wide lens or a panoramic feature on your camera. There are a few spots where people congregate every night to check out the light show of Hong Kong Island. These spots while busy, are well worth dealing with the crowds to get the great view.
The most costly, in fact the only one that will likely cost you money, is Victoria's Peak. This view doesn't require you to take a tram, but it's suggested as the stair climb is pretty intense. Once up on the peak you can either pay for a "premium" view, or wander around and find equal quality views of the city at night. Keep in mind that everybody knows that the night/sunset view are prime times to be there, so expect a lineup around that time at the tram station.
If you want to enjoy a great view of the city for free, the Kowloon waterfront is the place to be. As long as you're near the water you should see something, however the two best spots are at the lookout point near Avenue of the Stars, and near Sha Tsui Pier. The most popular, and arguably the best view is at Sha Tsui Pier, but it will be crowded at night.