There's a bar on the roof of one of the high rises in the CBD - I think it's called Altitude. You get a good view, it costs about $20 or so to go up there but you get a drink included in that (which is a better deal than the Marina Bay Sands rooftop where you get no drink and loads of tourists
People talk about Lau Pa Sat being good........and it is......but the trouble with it is that it's not that busy in the evenings and a lot of the stalls shut up shop once the business crowds have gone. The main exception being the statay stalls which open up around there in the evening.............great if you like satay (I don't). If you don't like satay best to go elsewhere to eat.
Chinatown may be busy - is it still the lantern festival? I think so - in which case it'll be pretty happening around there in the evenings at the moment. I think there's also a lantern thing going on around the river (Clarke Quay area) which could be worth a stroll along.
But most other things I can think of are mentioned on the link that SMS has posted.