I used to go to Singapore quite often on business, but have not been there since 1999. If memory serves, there is not much of the old architecture still standing; Chinatown, the Indian section and Raffles are all that I can remember. Everything else is quite modern, ranging from uninspired apartment blocks to spectacular new buildings downtown. Of course, as they are constantly building everything may look different by now. The night views across the harbor or from a well placed high rise building, are pretty nice.
For what ever reason, I never carried a camera while there, but I do remember thinking that it would be a street shooters paradise. My Singaporean business associates once told me that there were only two things to do there: eat and shop. Both of those activities, given the density of the population make for good street shooting, with an emphasis on the hawker centers (food courts). You'll have to check re any restrictions in these areas. One of the things that stuck in my memory was the Night Zoo; look it up.