Spent a lot of time in Singapore on business with many weekends between work weeks, so can surely relate. My favorite things to do include Satay Street at Lau Pa Sat on Friday or Saturday night, the Night Safari at Mandai Wildlife Reserve, evening light shows at The Marina Bay Sands and in the Gardens by the Bay, the National Botanical Garden (make sure to go in the paid orchid exhibit), and a stop at the Long Bar at the Raffles Hotel for an original Singapore Sling cocktail. If you have time before your departing flight, also recommend going to see the Jewel at Changi airport. The subway system is really nice and cheap - you can buy a day pass or just use your RFID-chipped credit card for paying individual fares. Hope you have a great trip!