Day trips from Ljubljana - would love suggestions

Where to begin...

• Bohinj, a wilder, less developed version of Lake Bled. Take the cable car up to Mt Vogel, if not for the skiing then for the wonderful views of Mt Triglav. There are also other ski resorts in the vicinity of Ljubljana, such as Krvavec.
• Piran on the seaside. Piran is a place I actually prefer to visit in the winter, when it's always warmer and sunnier than Ljubljana, the winter light really highlights the city's charm and there are fewer tourists on the streets.
• The Goriska Brda and Vipava Valley wine regions in the west. The western part of the country is such a contrast to the interior in the winter, around 10 degrees warmer, dry and usually sunnier with no snow.
• A trip to one of the spas in the east is also nice in the winter, for example Rogaska Slatina, Terme Olimia, etc. If using public transport, Lasko is easiest to reach.
• Unfortunately you will have gone by the time carnival festivities begin in the second half of February, but Ptuj is still a medieval town worth visiting, as is the nearby Jeruzalem wine region.
• Velika Planina, a plateau full of traditional shepherd huts. Alternatively, the Pokljuka plateau above Bled and Bohinj is also a popular destination for winter hiking, as are a myriad different hills around Ljubljana. If locals aren't skiing on the weekend, you'll likely find them ascending one of the hills surrounding Ljubljana. As long as there is an inn at the top and nice views, people will be hiking there.
• The medieval towns of Skofja Loka, Radovljica and Kamnik.
• Postojna and Skocjan Caves.

Further out, Venice is an option for a day trip, although a long one and you would really just get a taste of it, but still, the city in winter is a very different experience to the summer, when most people get to visit, fewer tourists and more locals around. It would benefit from an overnight stay though, as would the Soca Valley probably, which is a wonderful place to visit, but the short daylight hours and the long drive to get there mean you would have to start early. Then again, there are less sightseeing opportunities in wintertime, so you may find a single day sufficient.

And finally, this is more of a personal favourite, not really a tourist sight on its own, but still - on sunny winter days, I often go for walks on the clifftop Rilke Trail between Duino and Sistiana in Italy. Combined with a visit to the city of Trieste, it is a nice day out.

PS: This list is really just a guide of sorts, I encourage you to explore further and find places you would prefer to visit according to your interests. Besides, some of these places are quite similar to one another, so it would only make sense to visit all if you have a particular interest.

There's an 8:25 AM train from Ljubljana that's scheduled to reach Zagreb at 10:43. There are multiple late-afternoon and evening return departures with running times of less than 2-1/2 hours; some of those are by bus. I've only been to Zagreb in the summer, but I imagine it's still quite lively in the winter, and it has some good museums if it's a bit too cold to spend all your time wandering around the large historic district.

Thank you Dejan for all the excellent suggestions. We decided to get a car for a week of our time here so we can easily get to places like Piran, a spa, etc.

This suggestion is not a day trip out of Ljubljana but a food/drink walking tour that we really enjoyed when we spent 2 weeks there in October 2016. Tour guide took us to several places-many we would not have discovered on our own and were able to visit again. The website has all the information www.ljubjananjam.si
Very easy communication with Iva via email, scheduled tour before leaving on trip
Have fun, Ljubljana is a wonderful city.

I think the link is
http://www.ljubljananjam.si/food-walks