The decision
Door: overnight berth train from Hanoi station, or fly/transfer via Noi Bai — decide by timetable. Same-day return is exhausting; do not jam it into a 3-day Hanoi. This station only notes the decision — Sapa street sights stay off the Hanoi graph.
How to close on return
Plan from two days one night; taking Sapa means dropping a West Lake day or keeping Ba Dinh to exteriors. No Beer Street the night you return. If Hanoi time is short, prioritise Hoan Kiem + a mausoleum morning and leave Sapa as a separate add-on trip.