Free Tour from Changi
If you have over 4 hours of transit time, you can go for a free tour of Singapore from Changi airport. http://www.changiairport.com/changi/en/airport_guide/exp_changi/rest_n_relax/free_sgtour.html?__locale=en
Promenade and Merlion
The closest MRT station to these 2 sites is CityHall. From Changi Airport, the ride will take about 40 minutes. The walk from the MRT station to Promenade is approximately 15 minutes.
This is a historic hotel in Singapore. Don't confuse it with Westin Raffles hotel right across the street. The Long bar of this hotel is where the "Singapore Sling", a cocktail, is originally concocted. They give you peanuts as you wait for your order, and you can throw the skin on the floor. Pretty much it is the only place in Singapore where you can litter without fines! (Or, caning...). If you have Singapore Sling at the Long Bar, be ready to fork out about 25 Singapore dollars for the drink and tips.