Zócalo, Templo Mayor and Bellas Artes, with a long lunch and an easy evening.
Day 2
Chapultepec
Choose Antropología or the Castle, walk in Chapultepec, then dine in Roma.
Day 3
Coyoacán & Art
Timed Casa Azul visit, Coyoacán squares and a market lunch; add Anahuacalli if time allows.
Day 4
Day Trip
Teotihuacán for ancient history, or a pre-booked Xochimilco boat for an easier outing.
Day 5+
Go Deeper
Book Barragán, browse San Ángel or visit Jumex; add Puebla only with a full extra day.
Honest thought: walk in the morning and keep indoor plans for September rain. Two days give a glimpse; five allow time for neighbourhoods and a day trip.