What do you recommend for a trip to Mexico/Central America?

I am planning on visiting a friend in Mexico City sometime this Spring. I would like to take some time to see other parts of Mexico, or other parts of Central America.

I don’t really have a budget or time frame, so nothing is out of the cards. I am a young woman traveling most likely with other young women.

Where should I go? Suggestions for day trips from Mexico City are appreciated too.