3 days will be perfect - we will be there for 3 days in just over a weeks time although we normally stay 4 or 5 days. There's lots to do without going up to KL - Chinatown, Arab St, Little India of course but if you want to get away from the tourists go across to Pulau Ubin a small Island off the coast near Changi -I mentioned over on the SIN board and someone kindly gave the name of the pier -you just take a bumboat across.
The food is one of the main reasons we love going there - suggestions for hawker centres - Gluttons Bay (on the Esplanade at night), Lau Pa Sat, Newton Circus and not Hawker but the East Coast Seafood Centre. I would give Clarke Quay a miss as I think it's a bit touristy. Other good food -if you like Indian -The Banana Leaf Apollo on Racecourse Rd and a Champagne Sunday Brunch at one of the big hotels - the Fullerton and the Four Seasons are particularly good but if you are only there 3 days maybe leave the brunch until next time as it does rather wipe out a day
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P.s don't forget to try murtabak. Can't wait for my first bowl of laksa!