My grandfather used to say: there are two types of people in the world: those who simply take trips (even if to other countries) and then those who travel (and take in the experience and, hence, grow).
Here's a possible itinerary for you:
Arrive at Guatemala City
Take a 40 min ride to Antigua, a beautiful place. Stay in Antigüa as your HQ.
On day one, explore Antigua, drink the coffee and eat chocolates.
Day two: hike a volcano, called Fuego
Day three: visit a coffee plantation
Day four: go to lake Atitlán and stay in San Marcos for several days. Take in the local festivities as there is always something being celebrated.
Day five: travel by boat in the lake to other towns, like Panajachel. Go to Hotel Atitlán for a great and hefty meal.
Day six: visit a women's cooperative in a town around the lake. Use the tuc tuc service to travel around as the drivers are also great tour guides because they know their towns.
Etc etc.