There's the free SIA Hop-on bus you could try,

http://www.visitsingapore.com/publis...op-on_bus.html

Or the free transit tour from Changi

http://www.visitsingapore.com/publis...__transit.html

As for the best one-stop place to visit -- it all depends on your interest. But I think taking the MRT in to City Hall would be a nice start, along the way you'll get to see the non-touristy parts of Singapore. From City Hall you can shop, check out Raffles Hotel, walk around and look at some of the old and new architechture in the city, then head towards the ethnic enclaves of Chinatown or Little India.

If you want to shop a lot, then a trip to Orchard Road proper (Orchard MRT), where you can find shopping malls like Ngee Ann City (Takashimaya), Wisma Atria, Tangs, etc.