I'd say that's perfect. We were there for 5 days, and we were just beginning to run out of unique things to do.
Musts I would say are:
Zoo and Night safari, obviously. I would say do both, entirely different experiences.
Orchard Road- Capitalism at it's zenith, you will love it or hate it intensely.
Chinatown. Very pretty.
Food. Hawker Centres etc. you can eat anything.
We really did not enjoy Sentosa. Even for Singapore that place was crass, and my wallet felt sore afterwards.
Paradoxically, as a city it is an amazing place for a night out, however you may have to sell a kidney to afford it. I went out for the night to Clarke Quay with SD$100 and had to stop and get another SD$50 out halfway through. But hey, you don't get to do it often, so sometimes you just have to say to hell with it.
Other than that just walk around and explore. It is unbelievably safe as a country/city, so just roam.
Incidentally, you mentioned some other SE asian countries, are you planning to go to Viet Nam? I absolutely loved it.