First of all, good on you for taking your Dad.

As for places to go, obviously that would depend on his interests and likes. That said, it might be useful to consider where Hong Kongers of his age enjoy going and congregating. The Bird Garden in Mongkok is surely one place, a local park (HK Park or Kowloon Park, for example) early in the morning to watch, or participate in, Tai Chi is another. Spending some time at the Promenade in the evening to enjoy the lights of the harbour is essential. If he enjoys going to the horse races, an evening at Happy Valley (if they're running on the nights you're there) would be terrific.