Lake Bled in the Julian Alps of northwestern Slovenia is a stunning place. I waited a day to travel from Ljubljana to Bled to avoid a rainy day and my decision totally paid off!
Walk the shores of the lake, take a boat out to Bled Island to see the Assumption of Mary church or walk up Castle Hill to Bled Castle (the original castle dates back to around 1011 with extensive modifications completed in the 16th century). Great views abound!
Lake Bled is easily accessible from Ljubljana by bus or car. It is worth a trip there for sure if travelling in Slovenia.