Justin,

You should plan to spend some time in the Highlands area of Guatemala. Antigua and Chichicastenango are definately "on the beaten path", but Quetzaltenango (also known as Xela (SHAY-la)) is one of my favorite cities ever. A little higher in the mountains, near Hueheutenango, there are a series of natural spas where you can rent cabins with semi-private pools. I can recommend specific, inexpensive but wonderful hospedajes in both Antigua and Xela.

While Xela isn't really on the gringo trail, there is a fair ex-pat presence there that can help you get involved in some volunteer activities that will allow the cultural exchange you're looking for.

Also, while Grenada, Nicaragua (on Lake Ometepe) is a bit touristy, it's entirely worth a bit of a stay.