I lived in SIN for several years, so this might be colored by my time there! I think 2 days in Singapore is plenty of time to get a sense of the place, it's really small.

As for your choices of KUL and CGK, I'd substitute either/both out with Saigon and Bangkok. I never really enjoyed CGK as a tourist destination and I think Bangkok is a bit more fun than KL. If there is one city I try to send my friends to in SE Asia it is Saigon. Also, if you really want to visit Indonesia, try Bali, it's a great place.