The two monuments
Jvari monastery stands alone on the hill where the Aragvi meets the Mtkvari — park, walk up, ten minutes of views. Svetitskhoveli cathedral is in town, fourth-century foundations, painted interior.
Three hours, realistically
Drive out 09:30, Jvari at 10:00, Svetitskhoveli at 11:00, lunch in old town at 12:30, back in Tbilisi by 14:30. It's the most forgiving half-day on the list.
Combine it with
Mukhrani winery (15 minutes west, proper restaurant) or a Kazbegi drive continuing north — Mtskheta is on the way to the Military Highway.