A hike, crystal clear blue waters and a waterfall, this seem to be great ingredients for a great day trip from Ljubljana. "The 1.6 km long Vintgar gorge carves its way through the vertical rocks of the Hom and Bort hills and is graced by the Radovna with its waterfalls, pools and rapids [...] The Gorge was discovered by Jakob Zumer, the Major of Gorje, and the cartographer and photographer Benedikt Lergetporer in 1891. [...] To one interpretation, the name Vintgar originates from German Weingarten (the vineyards in the nearby Podhom), whilst the other interpretation says that the gorge section reminds on the wine glass." (Source) 

Only 45 min away from Ljubljana, Bled is a wonderful day trip any time of the year. You can take a walk around the lake, climb to the caste or take a boat to the island.