Three focused days is the right amount
Singapore is compact enough that three days covers the essential experiences without rushing. More is fine if you want to go deeper; two days feels slightly rushed. Here is how I would structure three days.
Day 1: Marina Bay and the waterfront
Start at Marina Bay Sands Skypark observation deck (SGD $26) for orientation views. Walk to Gardens by the Bay — spend an hour in the outdoor Supertree Grove, then one or both conservatories if you want (Cloud Forest is more impressive than Flower Dome). The Supertree light show at 7:45pm is free and genuinely spectacular. Dinner at Lau Pa Sat hawker centre — the evening satay stalls on the surrounding street are a Singapore institution.
Day 2: Cultural districts
Morning: Chinatown, Sri Mariamman Temple, the Buddha Tooth Relic Temple (free entry, extraordinary interior). Lunch at Maxwell Food Centre — Tian Tian chicken rice is the famous dish, SGD $6. Afternoon: Little India (Tekka Centre market, Sri Veeramakaliamman Temple, the color and commerce of Serangoon Road). Evening: Kampong Glam (Arab Street, Sultan Mosque, Haji Lane’s boutiques), dinner at a Malay restaurant.
Day 3: Nature and Sentosa
MacRitchie Reservoir tree-top walk in the morning (free, primary rainforest in the middle of the city, genuinely beautiful). Afternoon: Sentosa island — beach if the weather is good, Universal Studios if not. Evening: Clarke Quay or Boat Quay for the riverside bar scene, or the rooftop bar at 1-Altitude for city views.