Beautiful old city
I loved Ljubljana, such a beautiful city. Being so small it has a very chilled and friendly vibe, and I was amazed that most people there (and in Slovenia generally) speak English. I didn't bother with the castle (once you've seen a castle you've seen a castle), but it's worth checking out the markets (everyday but Sunday) and, if you're into street art and graffiti, the student area. The city is very walkable, too.
Insider Tip:The free walking tour is a great way to get to know and see all the sights of the city.