Gardens By the Bay is top visited site in Singapore. Opened in 2012, this is a spectacular 250 acre nature park, consisting of three waterfront gardens: Bay South Garden, Bay East Garden and Bay Central Garden. There are also numerous indoor gardens and conservatories (Flower Dome, Cloud Forest), for which ticketed admission is required. It is possible to run among the outdoor trails, although it is crowded at certain times and might be frowned in the interior walkways. The best running – and where biking is permitted – is along the the waterfront promenade in Bay South and the perimeter of Bay East Interim Garden. One can do about 2 miles in the gardens.
A great run is to combine the Gardens by the Bay with Fort Canning Park — an iconic hilltop landmark has witnessed many of Singapore’s historical milestones. The hill once sited the palaces of 14th century Malay Kings and served as the Headquarters of the Far East Command Centre and British Army Barracks. This is one of the few places in Singapore to do some hills. There expansive lawns and lush gardens, great for running, and some good stairs. The Helix Bridge
There are lots of options here. Perimeter of Gardens by the Bay is ~2 miles, and Fort Canning is ~1 mile around. Use the Marina trails and the iconic Helix Footbridge as a connector. Make the run even longer by doing the Marina Bay loop.