I'm from Singapore but have lived in Australia for 10 years, although I've been back for a visit at least once every two years.
The ZOO! Not just the day zoo (which has an awesome water park, too -- don't go there with the kids until you've seen all the animals that you want to see, as it's quite possible that they won't want to leave the water play ground -- and bring bathers for everyone), but the Night Safari as well. There's also the new River Safari, which has PANDAS! I adore pandas. It only opened last year. So that's three aspects of the zoo right there. With the Night Safari, definitely pay the extra to get the tram rides, as some trails are only open to the trams and you won't get to see certain animals if you're only on foot.
Food. It's fantastic for a wide variety. Get to Crystal Jade for some fantastic dim sum (especially the Shanghai pork soupy dumplings, also known as xiao long bao). Hawker centres are also great for cheaper fare, although if you're Caucasian you will likely be borderline harassed by hawkers to buy their food -- just say no politely and keep going.
If you're walking down Orchard Road, you'll see some old men selling ice creams from their push cart -- usually $1 for a big wedge of ice cream stuffed in some rainbow bread, and it's delicious in the tropical heat.
If you want to splash out on accommodation, we stayed at Treetops apartments two years ago and they were fantastic, great location. Nice to have an apartment rather than just a hotel room, too.
And yes, taxis are cheap, but it can be a bit of a wait to get one if it's a rainy day, etc.
I'm personally not a fan of Sentosa because I think it's overall a little contrived (imported sand and all) but some people love it. I haven't been to any of the integrated resorts, either, but it's supposed to be great. If you google "National Parks Singapore" I think there's also a treetop walk (free) at one of the nature reserves, which I've been dying to do.
Singapore is fantastic, wish it was cheaper. We're going in May and I can't wait. My then two-year-old's head is going to explode when he goes to the zoo!