Even if you somehow managed to cover all of Sentosa what about:
-The SIN flyer.
-The world famous zoo
-Botanical Gardens+Orchid Gardens
-The River Safari
-Arab Street.
-Gardens by the Bay
-Skypark
-Orchard Street
-Sentosa cable car
-Chinatown
-Little India
-Flight Simulator Experience
-Hiking (The Southern Ridges, MacRitchie Reservoir)
-Light and Water Show
Not sure why anyone could possibly be bored in SIN unless they were there for at least a week.
I was there with my 5 kids last summer (tr might follow) for a week and must say, that you could spend a week there, but for a couple without kids and not leaving the hotel at 11am only less time is ok
Zoo and safari/ river cruise are very nice, but not a must do without kids. Not majorly different to a regular zoo
SIN flyer takes a hour max
Botanical gardens and gardens by the bay, cute, but not everyone is into plants
Chinatown can be had in NY as well
I can go on and on, but I would say 3 days is plenty. Sentosa is nice and fun