Days 1–3
Follow the 3-day itinerary as your base — Marina Bay, the Chinatown/Little India/Kampong Glam trio, and a Sentosa or culture day.
Day 4 — Katong or Tiong Bahru
A full day slower-paced neighbourhood exploring — Katong's Peranakan shophouses and food trail, or Tiong Bahru's art deco streets and café scene, depending on whether food or architecture pulls you more.
Day 5 — A proper day trip
Pulau Ubin for a half-day of cycling, the Singapore Botanic Gardens for a quieter green space, or a day trip across the Causeway to Johor Bahru — whichever fits the pace you're after.