The airport is fairly close to the city centre and transportation is excellent, but I would not recommend you to try to see Singapore in just a little more than two hours even though you may technically be able to get 30-60 minutes in the city centre.
Immigration is very quick though, probably the most efficient I have ever experienced in an Asian country.
Singapore Changi airport is an excellent airport, rated as the best in the world, so enjoy your time there instead.
There are so many shops, restaurants, a pool (if you prefer to go swimming), casino, and lots of other things to keep you well entertained for two hours.