You'll need to be a bit flexible depending on your interests and the weather - some walks and mountain passes will not be accessible. But a few ideas:
I'd suggest stopping at Škocjan caves on the way from Piran to Kobarid, and potentially Tolmin Gorge as well (though that is also doable as a day trip from Kobarid).
In Kobarid, there's the WW1 museum and the Kozjak waterfall (both recommended). You can also go canyoning or white water rafting from either there or nearby Bovec - we went from Bovec but I like Kobarid better as a town.
The Vršič Pass could well be closed during your stay (I think it was April?) so you'll need to work out a route from Kobarid to Radovljica. This trip advisor answer has an excellent summary.
In that area, I'd recommend going to Lake Bohinj and Bled (the latter is gorgeous if you're early enough to avoid the crowds), and Bohinj / Mt Vogel / Pokljuka are very underrated. There's an alpine dairy museum in Stara Fužina too.
For tips for Radovljica, Adele in Slovenia is a local.
On the way to Ljubljana, we stopped at Škofja Loka. Kranj would also be an option (though the outskirts are ugly). Plus you may want to visit Postojna / Predjama from Ljubljana.
In Ljubljana itself, we ate a lot (food tour + Open Kitchen + gelato), went to the castle, and wandered around near the river and took lots of photos.