Hanoi punishes the tourist who tries to rush it. Three days is the minimum that lets the city stop feeling like a checklist and start feeling like a place.
Day 1 — Arrive, reorient, walk
Land, drop your bags, walk immediately. Don't plan this day. The Old Quarter's street grid will confuse you and then reveal itself. Let it.
End the evening at Hoan Kiem Lake. Sit by the water. Watch the city wind down.
Day 2 — Coffee and the lake
Start at Cafe Giang before the morning crowd. Order egg coffee. Take your time.
Spend the rest of the morning walking the lake perimeter. If it's a weekend, the surrounding streets are closed to traffic — the city opens up completely.
Afternoon: wander north into the Old Quarter streets. Each one used to specialise in a single trade. Some still do.
Day 3 — Go slower
By day three Hanoi will have slowed you down whether you wanted it to or not. Let it. Revisit somewhere from day one. Notice what you missed.
Leave late if you can.