Ljubliana is a lovely city. Take the funicular up to the castle. Bled is less than an hour from there by car. If you can at all, spend a night there. Take the hike up to Bled castle if you have time. Prepare to spend a couple of hours doing it though. If you have time, take one of the Pletna boats over to the island in the lake. Again though, it's a couple of hours to get there, spend some time on the island and back.
Lake Bohinj is another almost equally beautiful lake about a half hour south of Bled. Well worth a trip even if it is a separate one. Triglav Gorge is also beautiful. Waterfalls and a lovely walk along the side of the river in the gorge itself. That's pretty much what we did as we only had 2.5 days there. There's lots more to do in that area though.
You could also head down to Piran on the Adriatic coast. We never made it but it is supposed to be worth a visit. The good thing about Ljubliana is that nowhere in Slovenia is too far. Have fun. I'm jealous...;)