I went to Bled as a day trip while staying in Ljubljana. It is a beautiful place and I consider it a most if you are in the area.

I would say that it is very easy to visit it as a day trip. I drove there (45 mins from the capital) and it has a scenic road which surrounds the lake. You can make a several stops to enjoy the amazing views of the lake. If you follow the road you can also visit the Castle, which is really nice and with great views!

Overall, I spent ~4 hours here and it was one of the highlights of Slovenia, which I consider a beautiful country!

Enjoy!