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.
The little Island on Lake Bled can be reached by boat, but the best part is the view you get from the shore. Even on a late cloudy day, the place looks very nice.
They even put a little heart for pictures ;)
The walk to the castle is doable for everyone and it offers some very nice views. Below you can see a picture from next to the lake.
The first mentions of the medieval castle date far back in 1011 and is considered to be the oldest castle in Slovenia.
The oldest part of the castle is the tower that is built in Romanesque style.
Walking down from the castle, you can spot the boats at the end of their 'shift'. These boats are called Pletna boats and are made by locals. "The origins of the Pletna boat go back to 1590, whilst it size allows safe transportation of 20 persons. [...] The Pletna boat is a wooden flat-bottom boat with a pointed bow and the stern widened with a step to enable passengers to enter. The boat is propelled with the special "stehrudde" technique where the oarsman is standing and rowing with two oars. "(Source)
Don't miss this wonderful day trip.