There's a passenger ferry that runs between the old Kai Tak cruise terminal and Kwun Tong that gives you amazing harbor views for HK$2.50. Runs every 20-30 minutes and takes about 15 minutes each way.

Most tourists never discover this route because both terminals are off the main tourist areas, but the views of Hong Kong Island from the water are better than Star Ferry since you're further out in the harbor. Plus you get to see the Kai Tak cruise terminal which is pretty impressive architecture.

Combine it with exploring Kwun Tong's industrial art galleries or just use it as a cheap harbor cruise. Ferry runs roughly 7am to 8pm daily.