Singapore really has only one airport, the highly rated Changi International Airport. The airport has consistently been voted as one of the world's favorites, year after year. If you have to have a layover somewhere in Southeast Asia, this is the place to make it. You may not want to leave!
Technically, there is a second airport in Singapore, but Seletar is only used by one scheduled airline, Berjaya Airlines, for flights to Tioman and Redang islands.
For more information about what to see and do in Singapore, see the on-line Asia for Visitors Guide to Singapore.