Mexico City doesn't need to be exhausting. Take your time, and you'll enjoy an incredible trip.
9:30am: Chapultepec Park
Let's start this trip off nice and easy. It may be the biggest city park in Latin America, but you don't have to tackle all of it. There are trails and a good dozen museums, but you can take it at your own pace and still find a lot to enjoy.
11 am Chapultepec Castle
A ways up top, still in the park, is the national history museum and a big castle from the late 18th century. Watch for the train at the bottom of the hill (pictured above) and you'll see it's a simple trip.
1 pm National Anthropology Museum
Guess what? Just across the street is one of the most famous museums in the world. Visiting doesn't have to be too intense, and especially on weekdays, it's a fascinating place to learn about Mexican history and culture. The restaurant is excellent, and highly recommended for lunch, before or after strolling the galleries.
Alternatives: Second-Time visitors to Mexico City may have covered Anthropology already. Conveniently, there's a whole world of things to do in the immediate area. From the Rufino Tamayo Museum, to the restaurants and eateries around the Lincoln Park Section of Polanco.