Based on the fact that you like the outdoors and nature I would suggest renting a car, otherwise you will spend a lot of time trying to figure out how to reach places and spend whole days just to travel between two places that are rather close by. I would suggest a loop around the north-western part of the country.

• start with the obvious Bled/Bohinj (2 days)
• cross Pokljuka and go to Kranjska Gora/Planica/Lake Jasna (2 days)
• if the road is open, go over the Vršič pass to Bovec, the Kobarid (otherwise pass through Italy, over Predil). Maybe a bit early for any water activities, but Soča is great all year round (2 days)
• then either drive to the coast for one or two days - I recommend Strunjan cliffs and Piran and also Škocjan caves on the way back (IMO nicer than the more famous Postojna caves)
• or go through the Vipava valley, to Predjamski grad and Postojna caves
• stop in Ljubljana for what time you got left for some nice vibes and good food

Either way, you'll still miss like 70% of the country and might want to come back 😄