April–August in Hanoi is brutally humid. Do walking tours before 10 AM or after 5 PM. Midday is for indoor cafes and naps.
Locals nap from 12–2 PM — many small shops close. Embrace the rhythm and you'll enjoy the city more.
Hydrate constantly. Coconut water (nước dừa) sold from street carts is a refreshing electrolyte option.