No need to purchase an Octopus Card for a short stay. Well, when you arrive Hong Kong Airport, please take the bus S1 and get off at Tung Chung. It just take you about 20 minutes and HKD3.5. The bus stop is just at the noted Citygate Outlets.

Then, just in the shopping plaza, you could catch the metro of Tung Chung Line and then transfer to Tsuen Wan Line at Lai King. Please get off at Jordan station, and take the Exit to the Nathan Road. Walk down along the road, you will find the Granville Road on your left side. Finally, you could walk along this road until to Hong Kong Science Musuem, where the bus A21 could drive you back to the airport in 1.5 hours.

Alternatively, you could get off from the metro at Tsim Sha Tsui station, and then take the Exit B to the Granville Road firstly. After visiting this road, you could come back to the Nathan Road, and walk up along it to Jordan. Then, please hire a cab driving to the Kowloon Station, where you could catch the Airport Expree train back to the airport in 20 minutes.

Anyway, please refer to a Hong Kong map to arrange your trip properly.