All the four terminals in Singapore are interconnected through a shuttle.
You will have enough time to explore the entire airport which is wonderful.
Goto Terminal 1, Level 5 where Jewel Changi is there with an awesome man-made waterfall.
The Canopy bridge, canopy park and many more activities are there in Jewel Changi which have an entrance ticket that can be purchased online through Klook, GetYourGuide etc sites or even at the counter.
There are a large number of Lounges where your Priority Pass / Dreamfolks etc can be used for lunch / dinner.
Passing time is not an issue and as long as one doesnot leave the airport to go to the city, you donot require a transit visa.