Guatemala City is the capital: its historic city centre has a few buildings and museums that allow for an interesting view on the Maya civilisation. It's possible to take many excursions departing from this city that is surrounded by several volcanoes. For its part, Antigua has been designated as a Humanity World Heritage for good reason: the ancient capital is full of places with a prestigious past. Climbing Pacaya Volcano is one of the possible hikes.
Take advantage of a Thursday or Sunday to go to the Chichicastenango market. Although it's very touristy, it's a unique place where you can see all the local crafts. A little farther out, Lake Atitlán is a relaxing place. Beautiful hikes around the lake allow you to discover the villages and the surrounding areas.
In the north, Florès is a beautiful place to explore the Péten jungle and the archaeological riches of the Maya civilisation. Tikal is the the most impressive Mayan site and should not be missed under any circumstance.
On the Atlantic Coast, Livingston, a Garifuna village, will add a Caribbean touch to your trip. Go back up the Rio Dulce to Lake Izabal where you can observe the wildlife.
On the other hand, Puerto Quetzal is the largest port on the Pacific Coast. Many water-related activities are possible on the surrounding beaches.