Garni and Geghard are located very closed to each other but they are very different.

Take public bus no. 5 or trolley bus no. 1 (100 DRAM) in the center of Yerevan and get off at Palace Trade Center bus stop. There are marshrutka station to Garni (250 DRAM) at Palace Trade Center. It takes 1 hour to get to Garni from Yerevan.

Garni Temple
Garni Temple

Temple of Garni was dedicated to ancient Greek god Helios and was built in I century AD This is very unique building for all Caucasus. The temple is very well maintained and should be visited. There are ruins of old church, walls and ancient bath next to the temples.

Geghard
Geghard

We took a taxi (3000 DRAM) and went to Geghard. It is a religious complex built in the 4th century. It was founded by Gregory the Illuminator. You should read about him before traveling to Armenia: http://www.vehi.net/istoriya/armenia/agathangelos/en/AGATHANGELOS.html . There are a few religious buildings curved out in the rock. The complex is surrounded by walls. The main church (4th century) is the most significant building of the complex.