6 hours is plenty of time.
My instructions:
Walk towards Immigration.
Show Passport and Immigration Form.
Smile.
Enjoy the smart alec comment you'll get if your passport photo doesn't match you.
Jump in a cab.
Ask for one of the Hawker markets (best to choose one beforehand based on food type preference, and have read up on Hawker food market etiquette before travelling).
Choose stall, order food, enjoy food.
Travel Back to Airport.
Back through Immigration.
Or, they should consider checking out the Singapore Night Zoo. It's touristy, but not too bad. Not too expensive either really, including cab fares etc.
Alternative #2, head to the Raffles Hotel for a Singpore Sling (and perhaps some more drinks) and the bragging rights that they've had one!
Alternative #3, just stay in SIN and check out the movie theatre, walk around and perhaps the gardens upstairs.
Alternative #4, the sky wheel (
www.singaporeflyer.com.sg) has opened ("The world's largest observation wheel'). Could be worth a visit, followed by a vist to the various quay's for a walk or drink, before heading back