Blagaj is the Buna spring and a quieter bed than central Mostar. Prenj and Čvrsnica are the Dinaric massifs above: karst, exposure, and days that eat fitness. This is the mountain half of Herzegovina. Come for ridgelines and via ferrata, not for a café crawl.
Ridges, ferrata and the spring
Prenj hiking is committing. Loose rock, weather and navigation. Čvrsnica is the same family of problem. Via ferrata is the equipped version of that exposure. The Buna is the easy, beautiful morning if your legs are cooked. Don't stack a 12-hour ridge the day after high-water rafting.
When to go
Avoid midday summer heat on the karst. Snow lingers. Open the Book here tab for guided mountain days in Herzegovina and Neretva.