I can offer you a lot of suggestions. I'm assuming you're thinking of four cities, including CDMX, about a week in each.
Would you want to rent a car and drive yourselves? Or take buses/coaches? Or fly? Mexico has an excellent network of high-quality private coach companies, so if you want to travel by land, that's a good way to go. But Mexico is much bigger than many people realize, and distances can be quite long, so flying is also a good option.
Besides CDMX, you should hit Guadalajara, and Guanajuato. You could easily do those three by bus. The fourth city I'd recommend is Oaxaca, you'll want to fly there. I think those four cities will give you a great chance to experience Mexican culture and gastronomy and see some really great colonial architecture. Plus, from Guanajuato you can make an easy day trip by bus to San Miguel de Allende, a really charming city (and a UNESCO World Heritage Site). And from Guadalajara, you can bus down to the Lake Chapala area and visit us immigrants. Ajijic, where I live, and the surrounding towns on the shore of Lake Chapala, are home to the largest expat community in the country.
I have to say, though, that spending a week will require you to be creative about filling your time. There are plenty of things to see in each city, enough to easily fill a few days, but a week would be a long time to spend in Guanajuato for sure
Let me know if you have any other questions or need different suggestions.
