Like C-Kay posted earlier, really depends on what you'd like to do in SIN. if it's some heavy duty shopping, then 2 days might be a bit short, but if it's just to stroll around and take in the sights, 2 days should suffice.

CGK also is not really a major tourist destination. Better to head out to Bali to kick back and relax for a few days. Booked in advance, you should be able to snag some cheap tickets on one of the many LCC's.