I am travelling to Mexico for a week at the end of April with a friend of mine, both women in our mid twenties. We are flying into Mexico City. This was a last minute decision to add a holiday onto a work trip to the US. As I only have a week I don't want to rush around but would really like to get some suggestions on a couple of places to visit on a round trip back to Mexico city to experience some of Mexico's beautiful beaches/snorkelling, markets and ruins/temples. I realise that it is spring break for many US students and would dearly love to avoid these crowds while still seeing the beach. Thanks for your help!

Spring Break ended more or less the end of March and then it became the vacation time for Mexicans Semana Santa but for the last week of April things should be pretty slow. Mexico city has lots of ruins in the city plus just a few miles away. You only have a week so I suggest you see Mexico City and save the beaches for another trip.
I just got back from my umpteenth trip to Mexico, still managed to miss most spring breakers and had a great time. I have been to Mexico City before, though, so my side trip for 5 days to Merida was a fantastic and welcome trip. You decide -- the City has so much to offer and is truly glorious. You must of course take a day trip to Teotihuacan. Any hotel can help you book that. You must schedule your trip along a full Sunday since all the museums are free Sundays and the Anthropology Museum is a must as well as the National Arts Museum and the Museo de Arte Popular... The parks are teeming with life and the markets are fantastic. An entire week in the city might be too much, though, so you could - as I did - get a reasonable flight to Merida, which is a wonderful, small city full of great history. There you can take a 1/2 day trip to Uxlam and a day trip to Chichen Itza. The beach of Celestun is a short drive (and better than Progresso, which is more tourists and therefore, more vendors).
Or there are 1- or 2-day trips from the City to Taxco and Cuernevaca!
Enjoy!
If you try to add beach time to a Mexico City visit you WILL be "rushing around." A bus to Acapulco & back takes two days out of your stay.Do day trips from DF & save the beach vacation for the next visit...carracar

Consider Queretaro / Guanajuato / San Miguel de Allende.
Beautiful colonial cities, nicely situated for a weeklong trip on the bus. (IIRC, Mexico City to Queretaro is about 2.5 hours. Queretaro to Guanajuato about 3 or 4 hours. Guanajuato back to Mexico City about 6 hours.)
But no beaches.

Acapulco isn't a bad choice if you time it out right. Get on a very early bus, like 6am to avoid the traffic and to give yourself all afternoon on the beach, plus a night of clubs/bars if you choose. Then spend much of the next day on the beach and catch a bus back to DF. Its about the only real beach experience you could squeeze into that short of a trip. As long as you avoid traffic the direct bus should take around 6 hours.