January in Croatia

Pros: Zagreb in winter is full of warm cafes (and outside seats with rugs and heaters) and the Advent decorations may still be up. The museums like Naive Art and Broken Relationships (separate, not one caused by the other) and Mestrovic sculpture museum and so on will all be open. Chestnuts will be roasting by an open burner on street corners, and hot chocolate will be welcome. People will be dressed warmly and often well - old Zagreb families dress up. Young Zagreb will be denying it is winter. But it will be COLD. Try a winetasting in the Bornstein cellars on Kaptol street, right side,past the cathedral. Lovely couple, part Australian, gives great insights while serving an array of regional wines - you pick the region. Great way to start Croatia trip, my friends all love it. Not expensive.
Cons: Coastal towns are quite different out of season, as their workers have all gone home and many places shut. However, the cafes that remain open are local ones, with local workers. Dubrovnik is maybe still full on, but without cruise ships, which could go in the Pro column. New Year's parties are big in Croatian hotels, some of which reopen just for these big pre-booked events. Even Sarajevo hotels offer specials at New Years. However, Sarajevo is now heated by coal and wood stoves, and as it is in a bowl in the mountains, the air is like our forest fires at the moment. No fun. Also, the outdoor cafes and market mood of summer Sarajevo is not the same. Getting to Sarajevo from Dubrovnik is not so hard, and the valley roads are amazing at any time of year on a bus or minicab. Spend a night in Mostar if you can. The north of Bosnia route is very different. Flying back to Zagreb on Croatia Airlines would save you a very long bus trip on some very sad roads and scenes. Bosnia is still in recovery from war. Personally, I only went to Sarajevo in December once, and never again. The museums are still shut for lack of money, unlike Zagreb. The haves and have nots story of the Balkan war.
In contrast, the coast, especially as far south as you can go (consider Montenegro!) can be much warmer than the continent behind the mountains. Zadar would be fine at any time, as would be Split, Korcula, etc. Try to use Autotrans bus lines -see website. There are always cheaper alternatives, but those may break down or stop in odd places. Croatia Bus website shows most of them. I travel by bus in Croatia every year, and enjoy doing so. Buy your ticket and reservation number - carefully hidden on the seat base, if not on the overhead bins - well in advance of the last minute rush. I buy mine the day before. Keep valuables with you in a small pack, and put your oldest dark suitcase (mine is an RS carryon with wheels) under the bus at the start and pay the driver cash to do so. Usually costs 10 to 20 kunas, so have small bills handy to save swearing time. Wear shoes that do not slip on wet limestone streets! Enjoy!

Thank you for your detailed reply, it has helped in our planning. We are moving towards hiring a car. We hired cars in various parts of Italy last year and it went well. Just need to check out that we are not placing ourselves in any risk outside of the usual by driving between Sarajavo and Zagreb.

Hiya,

Early January is actually a great time to be in Zagreb because the famous Advent festival which starts at the end of November usually lasts until mid January. Christmas fair, with all the lovely food stalls and live music will continue after the New Year. The city keeps its buzz in the winter and it turns especially magical when it snows.
Quite the opposite, Croatian coast won't be as exciting as it is in the summer (except for bigger towns such as Split and Dubrovnik). So, I's day, don't be in a rush to leave Zagreb.