Almost every part of Singapore is accessible by train via her mass rapid transport system (MRT), so why not simply buy a train ticket and start your tour right away? You can save loads!

The Circle Line is Singapore's newest train line which circles the business-shopping-recreational belt that is located within the southern half of Singapore. You can have a nice little Circle Line tour if you know how to go about it. 

The Circle Line starts at Dhoby Ghaut  Station and ends up on its 29th stop at Harbour Front Station. It intersects with three other lines: North-South Line (red), North-East Line (purple) and East-West Line (green). This means that anyone travelling on the Circle Line can literally explore every part of Singapore by hopping on to the three other lines at these intersections but you will probably require a few days to do this.

If you don't have much time and want to sample various things here in a day or two, just hop onto the Circle Line and explore the various attractions around the Circle Line stations. However, not all the surroundings of the 29 stations along its route are tourist attractions. 

The Circle Line tour sieves out the best places. It will bring you through the city's art district (Esplanade), the Singapore Flyer & Marina Bay Sands Casino (Promenade),  a Singapore-styled public housing estate (Bishan),  an extensive catalogue of Singapore's flora (Botanic Gardens), Singapore's "Bohemian" or "Expat"enclave (Holland Village), a Chinese mythological theme park (Haw Par Villa) and Vivo City Shopping Centre (Harbour Front). You can shop at Vivo City while your spouse have a beer facing the harbour. And later, after dinner, you can walk across to Resorts World Sentosa where Universal Studios is located. 

The Circle Line Tour web site has a Complete Guide on where to stop along the route and what to see:

It's free!