If i buy Airport Express round trip ticket, must I buy octopus card to travel /shop in HK?

Dear Lina, the AE Round Trip Ticket can be used to ride on AE train for a round trip.

If you want to tour around the city or go shopping, you may buy a Standard Octopus Card. The card is applicable on AE train, MTR, Light Rail, buses, ferries, taxis, trams, etc. To get a card, each adult pays HKD 50 as the deposit, and HKD 100 as the stored value. Each child aged 3 to 11 pays HKD 50 as the deposit, and HKD 20 as the stored value. :)