The Mother of All Airports, IMO, easily the nicest place to spend a layover that I have been to. Full on spas, hotels, restaurants, bars, internet, free pc's, excellent transportation options to and from. I've killed several hours here several times, and it's about as good as it gets.

BTW, Tiger Air is now flying from a new "Budget Terminal", which is easily accessed by shuttle bus from the main terminals. The Budget terminal is not nearly as nice, and 90% of the perks at Changi are located inside the secured area, something OK for long layovers between flights, but likely not available if you're trying to check in super early for a flight the next day.

http://www.changi.airport.com.sg/

A couple things worth knowing

1) You can make free local phone calls from any public phone at the airport.

2) When you exit the airport and want to take a taxi, make sure you take on from the regular taxi queue, and do not take on from anyone who approaches you. These other taxis are $S30 flat rate to anywhere in Singapore. This will likely be more than double the regular taxi fare. Even if the regular taxi queue appears long it will move fast.

3) At any of the money exchanges you can buy a local simcard and top up cards.

4) The budget terminal is a huge step down, and in my min kind of an embarrassment to Singapore. However, the good news is that you can arrive as little as 45 minutes before your flight an checkin with no problem. At the other terminals you don't need to arrive more than an hour in advance unless taking a flight on an American carrier or taking a flight bound for the US.

5) Changi is absolutely incredible at getting you through Immigration, Baggage claim, and customs. It's rarely more than more than 20 minutes from exiting the airplane to getting into a taxi. That Rules!

6) If you like your drink and are staying in Singapore. Definitely stop by Duty Free and get some liqour. It's much much much more expensive in town. Hell call MongerX from a public phone for free and I will tell you what you can pick up for me if you don't intend to drink. hehe. You can bring in I liter of liqour, I liter of Wine, and three beers duty free.

Other info: Taxi rides between midnight and 6 AM require paying a 50% premium over the meter. Turns a 15 USD ride into a 22+ USD ride.