Ok, Guatamala is a freakin awesomely beautiful country, you've chosen well. I've never been to Puerto Barrios so i can't tell you of any things to do there, but there's plenty of cool things to do around Antigua. I would highly recommend hiking up the live volcano that's not too far from Antigua. Also, there are some crazy markets on every sunday i think that are about a 1 hour chicken bus ride from Antigua, they were pretty cool too. Lake Atitlan is only a few hours by chicken bus from Antigua, it is possibly the best place I've been to, the villages that surround the lake are just beautiful and a whole lotta fun. I don't know if you'll have time, but the Tikal ruins in the north of Guatamala are some of the best around, I would highly recommend visiting them if you can.
It's pretty hot over there year round so be sure to pack some summer clothes. I was in some pretty crazy storms while i was over there, i don't know if this is a common thing or not but i would pack a rain coat just in case. The people in Guatamala are pretty friendly. As in most parts of the world, they get friendlier the further you get from the major cities. Guatamala City itself is pretty rough, i wouldn't advise spending any time there unless you really knew what you were doing. A couple of the villages around Lake Atitlan do not tolerate tourists, so don't go to them, but San Pedro is as safe as any place in the world, the people are incredibly nice.
Also, English is pretty hard to come by over there, so take a Spanish phrase book with you.
Hope this helps.