As a Guatemalan physician I recommend you to be prepared for a couple of days of GI disturbance.
The best way to travel around is to have one of your local friends as a guide, he will know were and were not to go. The second best thing to do is to find a group of tourists (you will find a lots in the city of Antigua) a travel with them.
The lonely travel guide for central america have a fair description for the most popular sites to visit.
If you have more specific question send me an email, I will be glad to answer.
One last thing, dont bring any expensive jewelry, sunglasses, or electronic devices. Travelers checks and credit cards are a good option instead of cash.