Been to Singapore a couple of times. As already said, the cable car over to Sentosa island is great - and Sentosa island itself is quite a lot of fun, a kind of mini theme-park with beaches, gardens, shows - and the southernmost point in continental Asia (so they claim).
But my main advice would be: EAT!
The food in Singapore is fantastic, with excellent Chinese, Malay, Indian, and Indonesian food. Although they are a sanitised version of street food, I loved the hawker markets, where you can wander round the different stalls to get platefuls of loads of different dishes, and just find a seat at a communal table to scoff it! Little India and the Chinatown are also worth a wander around to find some interesting food.
Raffles is worth it for the experience, but the Singapore sling is vile, if you ask me, and extortionately expensive!