In the city
Hoan Kiem, French Quarter and Ba Dinh cores are walkable or short Grab rides. Cat Linh metro’s real tourist use: a Literature / south half-day, then walk from the exit — it does not loop the Old Quarter. Mind xe ôm safety and fares.
Airport
Noi Bai→city: airport bus or ride-hail to one end only (Hoan Kiem / Ba Dinh / West Lake). Prefer a car with bags; metro does not serve the airport. Halong/Ninh Binh via Giap Bat or hotel pickup is a full separate day — not an add-on after an overnight landing.