As always, the Night Safari and Zoo are wonderful. The night safari is a great (though gimmicky) idea; the zoo is the best I have been to.

For eating, Jiang-nan Chun at the Four Seasons (NOT the Regent owned by Four Seasons) is phenomenal. For only around 40SD on weekends, they have an "open menu" - order whatever you want from the list of 100 items. Service is great, atmosphere is excellent, and the food is the best Chinese I have ever had. For about 45SD more, you can get a free flow of Veuve Cliquot. If I had to recommend one single thing to do in SIN, this is it - it was the best couple hours I spent in Singapore.

Edited to add: I saw someone comment about hotels being a great price in SIN. This is very true, with the exception of Raffles (which is way overpriced), most places are very reasonable. Take the FS for only about 150USD; it is one of the most expensive in town. Although I haven't looked, I'm sure that you could get something decent for a reasonable rate. The Hilton you pass through from the FS to get to Orchard Road looked acceptable, as did the Grand Hyatt which is much like their Tokyo design(although don't eat in Straits Kitchen there) and Marriott which I passed from the outside.

Edited again: I just remembered something else. If you flew to SIN using Singapore Airlines, you are entitled to the Singapore Stopover Package. Go to their website for more info, but essentially it is a hotel, some admissions, and some coupons starting at just 54USD per night (hotels include Holiday Inn Atrium, Copthorne Orchid), 73 (incl Carlton and Traders), 98 (incl InterContinental, Hilton, Regent), and 150 (Raffles Plaza, Shangri-La, R-C Millenia). Prices go up slightly for subsequent nights.