I would take my overseas friend to East Coast Park in the evening. It may not be the most “touristy” place, but I feel it shows a very real side of Singapore. You can see families cycling, people having BBQs, friends chilling by the sea, and everyone just enjoying the breeze after a long day.
After that, I would bring them to a nearby hawker centre for local food like satay, chicken rice and sugarcane juice. To me, that is one of the best ways to experience Singapore: simple food, good company, and a place where people from all walks of life come together.