Any advice for a trip to Mexico?

My friend and I are 22-year-old women with quite a bit of travel experience (albeit in Europe). We’re both strong Spanish speakers.

We will spend most of our time in Mexico City (where my friend is living), but want to see more of Mexico. We’ll probably do a weekend in Oaxaca. We might even try to mosey on over to Guatemala.

Any advice on places to go, places NOT to go, and general safety would be much appreciated!