I agree as well. Singapore is exceptionally sterile and causes the anarchist in one to scream "
I just want to make something slightly untidy!"
However, I reckon 24-hours in Singapore is about perfect. You'll have done it and can say you have done it. That sounds like sarcasm, but it isn't. I think you've made the right choice.
It's been years since I last went there (for 48 hours!
) but I remember visiting some equivalent of Science Museum, and a bird sanctuary. The latter was quite good.
Otherwise my usual practice is to arrive in a city, point in a direction and walk (remembering not to cross the road until you have a green light, or you'll be fined!).