Do we buy an Hong Kong Octopus card and travel on the train or is better buying as we go?

Hello, we are staying for a week in Wan Chai and will need transport to and from the airport. We also intend to see Stanley Markets, the Peak, the Gondola and Big Buddha, Lo Wu, the night markets, the Star Ferry, Tsim Sha Sui, Disneyland and hopefully a harbour cruise on a junk. As we will be travelling on trains, MTR, buses and boats could you please advise as to whether we should buy an Octopus card or do something else?