Learn Spanish - There are dozens of schools to choose from.
Ixchel is a good one.
Costs $60-180/week depending on number of hours of 1 on 1 instruction. Schools can also organize homestays. Guatemala is one of the cheapest and best places to learn Spanish, but in Antigua you have to force yourself to practice.
Volunteer - There are lots of organizations where you can give your time
Proyecto Mosiaco Guatemala can help you find organizations throughout Guatemala for a small fee.
Camino Seguro/Safe Passage is a educational reinforcement program helping families of Guatemala City's garbage dumps.
Casa Alianza works with street kids in Guatemala City.
El Buen Samaritano is a project located in nearby Jocotenango.
Salsa Classes - Tons of dance schools and cheap lessons to hone your skills. Gloria is the best, across from Sky Cafe.
HikingPacaya Volcano - Available through most agencies. $5-10 for a big group tour.
Acatenango Volcano - Overnight trip giving you an early morning view of active Fuego Volcano.
Cerro de la Cruz - Short walk to a nice view over the city. Led by local police departing from the Municipalidad.
Visit a Village - Go with a group to the village of Santiago Zamora.
Great tour leaving weekly from Proyecto Mosaico offices.
Nightlife - Antigua has a lot of night life and really gets going on weekends when people come in from the City. Things end at 1AM but there are often afterparties somewhere.
La Sala - best place with live music on weekends
A Sin Ventura - salsa on tuesdays on a cramped floor with a latin mix on weekends
Toreros - salsa on thursdays. located behind the market
Red's - English pub with free pool