Are you surprised to read Zagreb here? As Zagreb is located in the inner part of Croatia it is the perfect city break during winter.
The bars, cafés, and restaurants are still open, the city is less crowded from the tourist point of view, but the student life is still vibrant and the Christmas Markets in Zagreb is super popular and gorgeous to visit.
On the Christmas time in Zagreb, you can experience a more local Zagreb than you would during summer.
Find flights to Zagreb with Skyscanner and Kiwi. It is actually (usually) the cheapest airport to get your Croatia trip started.