Roam in Changi Airport for an hour or two when you arrived.
Visit Orchard Road in the evening.
Eat chicken rice in Tian Tian Chicken Rice at 1 Kadayanallur St, #01 -10/11, Maxwell Food Center, Singapore 069120.
If got nothing better to do, eat Chwee Kueh in Tiong Bahru for me at 30 Seng Poh Rd | “#02-62 Tiong Bahru Market”, Singapore 168898.
Try to take MRT (subway) and tell us how it compares with HK’s, Japan’s and SF’s.
If still got time, visit any MRT station both inside and outside the station, tell us how organize is it 
Can combine with trip to Maxwell Food Center or Tiong Bahru Market.