The Jewel at Changi Airport, Singapore
If you have some time to spend at Changi Airport, be it that you're in transit or that you have to leave your hotel early, "The Jewel" might be an option. This is primarily a shopping mall, but there are also restaurants, cinemas, and even a hotel. For those with kids or just looking for some action, there is a maze, walking nets, and more.
The main attraction, however, is the world's tallest indoor waterfall, or rain vortex as they call it here. It doesn't operate 24/7, so if you don't want to visit during store hours, check out the homepage [↗].
Other than the activities, admission to the entire building is free.
"The Jewel" is located between the check-in areas of Terminals 1 and 3, so if you're in transit you have to immigrate to Singapore and go to passport control on the way back. However security is on the gates anyhow, so there is no difference to staying inside the transit area for that.
Of course, you can also visit "The Jewel" without a flight in mind. The airport is connected to the metro system, and if you have a day pass, it doesn't cost you anything extra.
