A lot depends on if this is your first time in town, what you want to see, and how comfortable you are getting around with the transportation (which has to do with #1). Knowing absolutely none of that... I would take the AEx to Kowloon station, taxi to Peninsulsa for some afternoon tea (or IC if you can see across the harbour). Walk around TST and neighboring area, grab some street food for dinner in Mongkok, cab to Kowloon station, then AEx back.