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