You wouldn't know it at first but this small city state has an amazing number of activities to cover. Sentosa Island has many amusement park related activities. The beaches are also a plus on the island. Although not comparable with beaches in other spots around the globe, the beaches provide a nice change to the metropolitan areas that cover most of the city.
There's still green to see when you drive a little bit out of the city. You'll also find Singapore Zoo and Night Safari there which are top class. Back in the city, Gardens by the Bay is a recent addition to the city but offers some nice sights and walks.