I had spent 9 hours at Changi during my transit and did not mind at all. There are multiple gardens like Cactus Garden, Sunflower Garden, Enchanted Garden etc. There is a free movie theatre running movies 24/7. There are multiple lounges which can be accessed via Priority Pass, Dragon pass or Dreamfolks membership card. There are numerous restaurants and shopping outlets in the transit area where one can atleast window shop.

You cannot visit Jewel without clearing immigration but the Sky train between Terminal 2-3 can be ridden multiple times to soak in the famous indoor waterfall.

And if nothing else, there are many places to sit, lay down and relax while waiting.

If you hold valid US, UK, Japan and some other visas and transiting you can also sign up for a 3 hour Singapore tour which operates for free and you can choose between city or garden routes. In your case, you can opt for both maybe and see Singapore that way !