There are two bus stations which connect Catemaco to bigger mexican cities. The ADO terminal, which services buses to the city of Veracruz, is located at the Eastern end of the Malecon, and is about 10 minutes' walk from the Zocalo.
San Andrés Tuxtla - A nice, real mexican city with less tourism. It can be reached by bus.
In the west you can find a station for the rural taxis or Mixtos. Alternatively you can flag these down at any point along their route. Some of the many destinations are:
Sontecomapan - This is a village next to a beautiful lagoon about 20 kms northeast of Catemaco. It will cost 10 pesos to get there by rural taxi. Once there you can take a boat or 'lancha' out to the cost 'la barra' for 40 pesos each way.
Montepio - Montepio lies directly at the beach of the Gulf of Mexico. Be aware that there is no Supermarket, Internet or ATM available.