The beautiful Truso valley, source of the Tergi River, heads west off the Georgian Military Hwy 17km south of Kazbegi. It's dotted with ancient towers, abandoned Ossetian villages and strange mineral phenomena, notably the Abano mineral lake which bubbles with carbon dioxide. Take your passport: the South Ossetia boundary is close so there are Georgian soldiers in the valley. They may stop you going beyond Ketrisi, 14km up the valley, or you may able to go as far as Zakagori, a further 3km.
A return taxi from Kazbegi (including waiting time while you explore) costs about 100 GEL to Kvemo Okrokana, 4km up the valley, or 150 GEL to Ketrisi. Mountain biking is also a popular means of exploring Truso.