The monorail from VivoCity on the third floor of HarbourFront Station to Sentosa. I think it cost between 8,000 and 9,000 won for two people. We went on a weekend, and the line was incredibly long. So we just walked in and used the trains inside Sentosa. If you're sensitive to heat or something, I recommend taking the monorail. It was a bit of a walk, and even with the shade, it was still tiring.