I spent some time in antigua and i loved it. The city is beautiful and fun. There are allot of students in town studying spanish from all over the world. Each night everyone goes out to a differnet bar. and they have kareoke as well.
Definately head up to Lago de Atitlan. There are two amazing towns there. One is San Pedro, this is more of a hippy party town. The other is San Marcos, which is a new age town. Both are awesome and the lake is beautiful. They are only about an hour and a half from Antigua.
Good luck.