just found out I have a full day in HK. Staying at the airport Marriott.

Oh, I would suggest Tsim Sha Tsui area to you. You can take Airport Express to Tsing Yi, where transfer to Tung Chung line and get off at Lai King. There transfer to Tsuen Wan line to Tsim Sha Tsui directly. You can also take bus no. A21 from HKIA and get off at Middle Road, Nathan Road, Tsim Sha Tsui.

In that area, you can have a good sightseeing at the Avenue of Stars and Victoria Bay. You can also experience the star ferry on the bay. For shopping, the Ladies' Market and Harbour City will meet your demand. The nearby dining is also easy to access.