Three days is the shortest version of the south that does not feel like a checklist. Day one takes the city's landmarks — the Reunification Palace, the Central Post Office, Notre-Dame — and then the Cu Chi tunnels in the afternoon, which is the pairing most people want and few itineraries get right.
Day two drives down to My Tho and on to Can Tho, with the river and the islets in between, and day three starts early on a boat to the Cai Rang floating market before the run back to the city. There is a shorter two-day version that turns round at the delta instead of going on to Can Tho.