The terminals
Four terminals, connected by a Skytrain — check which one you're departing from, since some involve a genuine walk or transfer. All are connected directly to the MRT's East West line.
Jewel Changi
The domed complex connecting Terminals 1, 2 and 3, centred on the world's tallest indoor waterfall (the Rain Vortex) and a canopy park above it — worth visiting even without a flight to catch.
On a long layover
Free transit tours are available for layovers of a certain length if you don't want to clear immigration; otherwise, Jewel and a quick MRT ride into the city are both realistic within a 6+ hour window.