I love Singapore and try to spend as much time as possible there. In saying that, I have never spent more than 2 weeks there at a time. I have a few friends who live there so for me there is a lot of catch up time, but I also like to get out and explore.
It really depends what you want to get out of the trip.
Singapore has some amazing back-alley food places that are definitely worth a visit, as well as some very high end restaurants you cant go past. Chimes on Northbridge Rd is a good place for a Friday afternoon beer followed by dinner. And I really love My Humble House at the Esplanade - try the wheatgrass and mint/vodka.
A trip to Singapore should not be without an afternoon spent in Raffles cracking peanuts and drinking a cocktail. Singapore Sling of course.
Sentosa Island is a nice chill out spot - although not really my scene, I have been there with friends and spent a few days. There are some lovely resorts. night clubs and a man-made beach.
Chilli mud crabs on the East Coast. Red House is my favourite.
Night Safari, Singapore Zoo. Worth a visit.
Karaoke Bars - yes, you read that right. Even more fun when its not your own town.
A ride on the MRT. Dont ask, but a good time waster to "people watch" for an hour or two.
And of course, shopping. Again not my scene, but I have ventured into some lovely little air conditioned shopping malls for respite from the heat. And managed to pick up a few little treats. about 6 years ago