Past reviewers have mentioned the language schools in Antigua. I studied for 2 weeks at Proyecto Linguistico Francisco Marroquin, which I highly recommend. Having only had an introductory course in Spanish in the 8th grade, I was a bit intimidated my first day of classes but by the end of my stay, I was giving people directions in the city! (Okay, it also helped that I was fluent in Tagalog and that many people would mistake me for a local). I also stayed with a host family who were so kind and hospitable. I still keep in touch with them today.
Places in Antigua to explore include La Merced church ("the yellow monster," as someone called it) and Casa Popenoe.