Under 6 hours — stay airside
Jewel Changi alone is worth the time — the Rain Vortex, the canopy park, and enough food options that leaving the airport adds risk without much upside.
6–12 hours — a Marina Bay loop
Clear immigration and take the East West MRT line straight into town. Merlion Park, Gardens by the Bay, and a hawker meal cover the essentials in a tight loop back to the airport with buffer time to spare.
12–24 hours — add the city
With a full day, add Chinatown for food and temples, or Kampong Glam for a different pace. Book a locker at the airport for luggage rather than dragging it around, and build in at least 90 minutes of buffer before your next flight.