I am travelling SYD - SIN - LHR at the end of January (scored Suites First class!) and have a 15 hour layover arriving at 5:35PM and departing at 9AM the next day. My idea of a good time in this context is a rooftop bar with a great view of the city and having a couple drinks; I don't really want to run all over but might go see the Merlion or other touristy type place just briefly if appropriate. I have a bundle of Mariott points but can't seem to figure out if there's one not too far from the airport that would offer that, or if I should splurge on something non-Marriott. I also have an AMEX plat so could use the "Fine hotels" program.

Suggestions on where to stay, which places offer such a bar (hopefully the same), and how to transit (cab?) Would be greatly appreciated.

You can stay on top of Marina Bay Sands - these rooftop areas on the 55th floor close at 4AM and then use the remaining time outside MBS/Marina Bay.

As far as of 'couple drinks' - make sure that you have an adequate budget for the rooftop bar at MBS - the cheapest drinks start from $30.

Several years ago I wrote here on a thread 'what to do in Singapore if you have 24h' - you might find that useful.