8-10 hours in the Jewel sounds a bit boring.
Why don't you take a taxi to Changi Village, have a walk around the parklands and beach, maybe a ferry across to Pulau Ubin, eat a few lunch courses at the Changi Village hawker centre and the eating houses on the opposite street, maybe a Tiger beer or two, then taxi back to the terminal (for a shower and change of clothes). Or, Joo Chiat Rd and Geylang Serai - some interesting Peranakan architecture in this area, and great food.
Much more interesting than looking at a waterfall and lots of shops.