Oh my goodness you are going to love it! Honestly the random wanderings led to some of our favorite experiences. The people are so lovely and the country is gorgeous.
Just go with a willingness to experience whatever happens!

We did a very similar trip a couple years ago and loved it. I assume you have Vintgar Gorge and the various other waterfalls and popular sights on your list. One maybe less popular place we thought was worth a visit was the Peace Park above the Soca River. Also we decided we needed lunch on our way south from the Soca Valley and we pulled stopped at a random restaurant (Fontana) in a random town (Kanal) and it turned out to be really good.

Hiša Franko is the opposite of a hidden gem, but it was worth it!

Doing a 10 day road trip through Slovenia next week and looking for any hidden gems or favorite restaurants/activities.

My route is - Lake Bled, Lake Bohinj, Triglav, Bovec/Soca River, Postojna Cave and Predjama Castle, Piran/Koper, Ljubljana

Thanks!