I'm one of those travelers who has to find some type of cultural artifact to bring home with me from my adventures. Not a "souvenir," per se, but an item that meant something to me during the trip. Typically this ends up being a local's piece of art.
While in Guatemala City, I found Galeria Arte Viva, a tiny gallery that sells regional works in a variety of mediums. They have hundreds of paintings, drawings, and sculptures for sale, and the staff are extremely knowledgeable on both the works available and the history of Guatemalan artwork. I definitely recommend stopping by, even if just to have a conversation with one of the gallery staff. They're passionate about Guatemala's cultural heritage and would be happy to talk with anyone interested.