One way of having a swift look around Singapore is to join one of the Duck Tours. These are quite fun as they do most of their touring on the river but also drive around part of Singapore central. Our hotel was close to Suntec City (a shopping centre etc) where the tour commences so it was easy for us to join it.
The Duck (previously used in the Vietnam war) leaves Suntec City and soon enters the water with a splash. It then travels sedately along in the water past various Singapore sights such as the Gardens on the Bay, the Singapore Wheel and the Singapore Merlion. There is a good live commentary and the Duck returns back the way it came (so whichever side of it you sit you get to see everything either on the way to the Merlion or on the way back) and then drives around the closer part of Singapore.
There is not very much leg room but it is adequate for the 45 mins or so it lasts. The trip can be combined with the hop-on, hop-off buses so you can see most of Singapore for S$33 (about £17). This is quite good value for the double ticket compared to other cities and I would recommend it.