Each time we visit, we include a Star Ferry journey between Kowloon and Hong Kong. This time we a return journey from Kowloon Star Ferry Terminal to Wan Chai Ferry Terminal at night. Although it was quite busy, we still managed to find a seat each.
I suggest purchasing the Octopus Cards when visiting Hong Kong, as the card can be used for payment on Star Ferries and Hong Kong buses, as well as the MRT and the Airport Express. If you purchase the Airport Express Return Ticket with the three-day unlimited travel cards, these cannot be used on Star Ferries and some of the buses.