Lake Bled will be a decent chilled base. You can do chilled out days at the lake or Bohnij lake close by or take day trips to cable car close to Bohnij or Vintar Gorge or Triglav National Park so plenty of options there. Bled Castle is worth checking out and I saw tobaggining and base jumping activities going on too.
Do check out Postojna Cave- fun mini train under the caves which are stunning itself. I think it is not too far off Trieste and doable as a day trip from Piran.
Easily my favourite country I have been to.