I would stay away from Guatemala City. There's really nothing there to see and it's not the safest city in the world. If you only have a short time, head to Antigua. It's not too far from Guate, but it's much different; it's somewhat touristy, but it's fairly safe, and among other things, you can hike up a volcano. If you have a little more time, take a getaway to Lake Atitlan and Panajachel. And if you have more than just a little time, get yourself up to El Peten and check out Tikal. That's a very long bus ride, though, or a short plane ride.