One of coolest things to do in Guatemala is riding on the local buses - called chicken buses which are "dressed up" big yellow school buses. If you're based in Antigua, you won’t find a cheaper way to get around than the chicken bus. Antigua to Guatemala City costs around 10Q ($1.25) one way, and to Panajachel (Lake Atitlan) costs about 20Q ($2.50).
comfortable luxury buses I've taken the chicken buses a lot im my travels through Guatemala and they are cheap but can get uncomfortable if you're on them for more than a few hrs. The other more comfortable alternative to chicken buses are coaches like Linea Dorada, which have bathrooms. Guatemala City to Tikal is about $20 round-trip.
If you want to go from Antigua to Panajachel, you should know that there is no chicken bus or other buses who do this route direct (except minivan shuttles). You'll have to stop in Chimaltenango, then hop on the hourly Rebuli bus which goes direct to Panajachel.