1) I've asked for -- and received -- a 6pm checkout once, so I think the hotel will try to extend check out by a couple of hours if it can. Having said that, it's all based on availability of course. One thing to note is that this hotel has a very nice gym and changing room complete with a sauna and jacuzzi, so you can shower there before you leave. I think the gym closes at 10pm though.

2) They've got different closing times at the lounge for weekdays and weekends if I remember correctly. Breakfast ends at 11am at the lounge on weekends and 10:30 on weekdays I think.

3) Public transport stops running at about midnight, so just take a cab. There're signs everywhere at the airport. Different cabs have different prices depending on the make of the car, so if they try pushing you to a Mercedes or some other fancy looking car, you can ask for a normal cab. Having said that, the price difference shouldn't be that great. At 3am, a cab should cost you about S$35 to the city at most.

4) I think most money changers are closed at 2am. Most taxis take credit cards, but check before you get on the car because there're still a few taxis that don't, or just drivers who prefer cash. The next day, if you would like some Singapore currency, just walk over to Far East Plaza next to the Hyatt or Lucky Plaza along Orchard Road and there're plenty of money changers at both malls. All large shops will take credit, but smaller family owned places and food courts don't.

5) Not sure what you like to do, but the Botanical Gardens is quite nice, as the OP above said. Gardens by the Bay is a also newly opened and you can consider going for a while if you're into big trees and exotic fauna. Otherwise, try visiting the different ethnic neighbourhoods in Singapore like Little India, Arab Street or Chinatown. It's an easy city to get around, and the MRT should get you to most major attractions.