Easy hike along the border, which will lead you to the territories of Carinthia, Slovenia and Styria. Between Stage 1 and Stage 3 you follow the Austrian long distance hiking trail 03 ("Südalpenweg"). Expect about 15km with 400m ascent.
N 46° 40.142' E 015° 00.584'
Stage 1: N 46° 38.709' E 015° 01.808': Hühnerkogel / Košenjak
Park your car at these coordinates if you come from Carinthia or Styria. I'm sorry I cannot suggest any parking when coming from the Slowenian side of the border (yet, suggestions are welcome).
You are on top of Hühnerkogel / Košenjak directly at the border between Austria and Slovenia. At a tree you will find a rather strange (german) sign telling you that you need to turn around, because the (Austrian) hiking path ends here at the border (although there are paths leading up here from Slovenia as well). At the same tree you'll find a red sign guiding to some destinations in Slovenian language. It has AB letters on it.
Now follow the narrow hiking path along the border towards the east.
Stage 2: N 46° 39.112' E 015° 03.788': Jancki rock / Janitschkifels
Close to this rock formation is the three corner point of Slovenia and the Austrian provinces of Carinthia and Styria. On top of the rock you will find two letters on the ground (on Austrian territory, less than 2m away from the borderstone). Convert both into numbers (A=1, B=2 etc.) and add them. This gives you CD. Note, that this may be hard to find if there is snow.
Stage 3: N 46° 38.982' E 015° 04.384': Sv. Urban / St. Urban
You are now at the St. Urban / Sv. Urban church. Go to the main entrance and count the number of iron nails in the door (they appear in groups of three, do not count the door handles). This number is EF
The Cache: N 46° 3(B+C).(B+F)41' E 015° 0(C-A).(B+D+E)10'
You will find the cache on the way back to the parking coordinates. Leave the main path, when it seems appropriate. The cache is hidden under a tree stump.