Stroll the lakeside and cross to Ngọc Sơn Temple for a gentle introduction to Hanoi's history and city center energy; the lake is open all day, the temple typically 8:00–17:00.
Return to Hanoi if flying out from there; for this plan, you will transfer back to Hanoi and catch an afternoon flight to Hue (allow spare time for traffic).
Cycle the flat countryside to see rice fields and an organic vegetable village where you can learn about traditional farming (morning tours run ~2–3 hours).
If you prefer staying local, get measured at a tailor for custom clothing (same-day options possible) or take an afternoon cooking class; classes typically run 3–4 hours.
Visit iconic French-colonial landmarks in District 1 for architecture and photo opportunities; both are open in the afternoon (exteriors always viewable).