1/ <<What can I do, is there airport lounges? >>
The best idea for you is to book airport transit hotel. A time slot of 6 hours cost from 76 SGD nett.
2/ The access to the airport air-side hotels depends on what airline you fly in and out of SIN???? Please specify...................
3/ <<Is it worth going into the city for a few hours late at night.>>
I would sleep as much as i can and venture to the city in the morning....for breakfast and better VIEWS.
4/ <<I presume I can leave luggage during the day and go for a tour.>>
Luggage question , ONCE AGAIN, depends on what airline you fly in and out of SIN???? Please specify...................
If your things will NOT be tagged through to the final destination the baggage reclaim area is at the ground floor, after Immigration and before Customs. Please check nearby information displays for the carousel assigned to your flight. You can also collect bulky baggage such as surfboards in the same area. Free trolleys are available next to each carousel.
The left baggage service temporarily stores your bags, coats or other items. Please clear Customs and follow the signs for "Left Baggage". The service is at Basement 2 for Terminal 1 and Arrival Hall for Terminal 2 and Terminal 3.
5/ if you fly full service airlines both legs, there is 99% chance your things will be stored in the airport hold during your stopover and be loaded to the next flight without your intervention.