Do you like museums, shopping, zoos? The main zoo is pretty cool and could easily absorb your day at the cost of taxis to/from the airport and a short stop to pick-up some food at a hawker mall (loved the food in Singapore - $4 produced some very tasty dishes)

For museums, I spent a good 3 or 4 hours in the Asian Museum (the one near the parliament buildings). Not as close to the truly "cheap" Singapore food but a nice location with nice restaurants close by. You should easily have enough time to escape the airport.