The most photographed mountain scene in Georgia — Gergeti Trinity beneath Mt. Kazbek.
Kazbegi is 155 km north of Tbilisi on the Georgian Military Highway — 3 hours by car in normal conditions, 4–5 hours if you stop at Ananuri Fortress, Gudauri viewpoint, and the Russia–Georgia Friendship Monument (all recommended). This is one of Georgia's most scenic drives and worth doing slowly.
The cheapest option is a marshrutka from Didube station in Tbilisi: 15 GEL, 3.5 hours, departures every hour from 07:00 until 18:00. No stops — you just get to Kazbegi. If you want the viewpoint stops, take a shared tour or private driver.