I would go to the lounge then take the one-hour, c.SGD2 train ride into town. Visit Gardens by the Bay while the trees are lit up at night. Grab dinner nearby (lots of options in one of the malls, the casino, or go more local to Little India or a hawker centre). Last train back to the airport leaves town at around 11pm (but double check that) or grab a taxi back.

Or you can take the last free evening tour but you need to register by 6:30pm, so I would do that before using any lounge. But that would mean eating dinner at the airport.

If you can skip the lounge on arrival, then I would also recommend taking a short walk from Henderson Waves/Southern Ridges to VivoCity/Harbourfront (sun sets around 7pm so plenty of time to do the walk) before moving onwards to Gardens by the Bay. Airport to Henderson Waves is about SGD20 taxi ride or a one hour, SGD4 train and bus ride.