Staying in Hong Kong for 3 days and more,travel the MTR and the Octopus card is a must. It is convenient and saves the trouble of carrying lots of coins around. You can get one from any MTR station.Every time the card is tapped,you will know the card balance and one can top it up at any MTR station, convenience stores like 7-11 and many supermarkets. Many stores also accept Octopus cards as payment for goods. The initial deposit of HK$50 for an Octopus card is refunded on surrender of the Octopus card.