Depend when you last visited Singapore.
You should revisit Sentosa, there are lots of changes since the coming to 'Resort World Sentosa' .... casino and Universal Studio
Others places might be of interest
Night Safari, if you have done the zoo but not the night safari, something might worth doing once
If you are into birds ... there are the ' Sungai Buloh Wetland reserved ', thou its toward the end of the migratory season now
Or Jurong Bird Park - bird in captive
Parks for casual strolling ....
Singapore Botanical Garden
Lakeside & Chinese Garden
For the more adventurous
Pulau Ubin - a layback and rustic island
Little India
esp on a Saturday night (be warned)
Enjoy your visit