Guatemala City is, quite frankly, not that interesting. What time do your flights arrive and depart? I might spend the whole time in Antigua and take one of those days to sign on with a tour to go to the market in Chichicastenango, about two hours from Antigua. Market days are Thursday and Sunday. The "tours" are really just your transportation to and from. You arrive in Chichi mid-morning and depart mid-afternoon. You're on your own to shop and sight-see in between.