Oaxaca in March

Hello! My girlfriend and I will be in Mexico for about 5-6 days in mid-March. We already know that we would like to first stay in Oaxaca City, but every travel guide I have read suggests that the town can be covered in about 2 days.

We would love to bus/fly somewhere else close (approx. 4 hours or less away) during our trip. We're not big beach people, but certainly aren't opposed to the beach as long as there is something else to do (ruins, shopping, wandering, restaurants, etc.).

Has anyone been to Oaxaca? Are there any suggestions to nearby destinations? We're very excited!!