1. Rum. The bars in Antigua and Guatemala City are full of amazing rum, I'd never tasted a Mojito like it
2. Visit The Plaza Central Park and chill out.
3. I recommend you stay at the Monastery not only a great hotel, but steeped in history and great food.
4. Cathedral de Santiago is picturesque inside and out.
5. The street markets are full of interesting cloths, food and jewellery. You d get a lot of people approaching you so be prepared to say no.
I was down to do a trek on one of the volcanic mountains, but my our son got sick with an ear infection his health was obviously more important. What I experience of Guatemala was amazing and is on my list to go back.