You'll quickly find when researching adventurous areas in Slovenia that if you want to go big and beautiful, you'll head to the northwestern quarter of the country, an area dominated by Slovenia's one and only national park, Triglav. And at the heart of this massive natural area ripe with 2,000 meter massifs is Lake Bohinj, better in so many ways than it's well-touristed counterpart to the north, Lake Bled.
A bold statement, you say? Consider the surroundings. Nestled in a valley in the midst of the Julian Alps, this 2 mile long, half mile wide lake is off the well-frequented highways and yet still accessible by bus. More tranquil and quiet, you may tackle the lush peaks encroaching on the water's edge or strike out for the greater, ice-frosted peaks of Lanževica, Mahavšček, and more. Slap Savica is a double-pronged tributary to the lake that falls a total of 78 meters and provides quite an appealing destination for a day-long hike from the small town of Ribčev Laz. It's a 7.3 mile hike that includes a variety of rich scenery at every turn. Then again, if hiking around the lake isn't your style (although you'll love the fact that you can take a dip in the summer-time wherever you like), take the cable car up Mount Vogel on its southern shore and finish the few hour hike up it well marked trails to the summit. Vogel is the source for quite a range of adventures beyond this, serving as both a popular ski area in the winter and the premiere take-off point for paragliding flights during the rest of the year. You can also even settle for a boat tour of the lake with a knowledgeable tour guide if you're more into getting to know the area.
There's a lot to do for the rugged adventurer in all of us without the sort of tourist numbers you'll see over at Lake Bled, meaning tranquility is more easily within reach, while rugged adventures most definitely abound. See the included links for more on what to see and do and where to stay when visiting Lake Bohinj.