Maybe when have the chance, should go there and see if we can catch any couples having PS, then ask them whether it was easy getting permission. Heh heh heh...
Lorraine, Changi boardwalk is a nice place to take sunset! There's a part near Changi Beach Club where there's a hut and a nice view of the boardwalk behind.
Near the hawker center where you can see Ubin is also a nice area, will have unrestricted view of sunset.
Hee hee... I live nearby, so I've done a few shoots there.
Some other places you can consider for PS is SBG. Lush greenery, large area, so with PG's creative flair, don't need to be worried that your photo will look the same as others.
I love cityscapes, so if you do, Shenton way actually has nice modern architecture.
Arab street has rather interesting and colourful shophouses if you like that too.
That's all I can think of now. Haha, I'm starting to sound like SPB tour guide.