Zagreb is the capital of Croatia. It is just a few hours of flight time from the most of European capitals.
I would recommend at least 2 days to visit Zagreb, starting with a morning visit to marketplace Dolac, followed with a walk to Jelačić´s square and the main street, Ilica.
In the evening you should visit one of the Croatian restaurants to enjoy the local cusine. Do not miss the Old town, St. Marko´s church, Stone gate (Kamenita vrata), the Cathedral...
There is a great vista of the Downtown from the Old town. Stop by in one of the great museums or galleries. If you have a few extra days, you could go to Hrvatsko Zagorje to visit some of the medieval fortresses and castles.