Europe in 1300
Southeastern Europe
Hungary
I put Hungary with the Balkans mainly because during this period the attention of the kings of Hungary was focused in that direction, though in later centuries they were concerned more with Poland or Austria. This was the most significant kingdom in the Balkans for most of the late Middle Ages. In 1300 it was still recovering from a terrible invasion in 1240-41 by the Tatars.
Serbia
Serbia was its own kingdom in 1300. It was one of several kingdoms established in the wake of the collapse of the Byzantine Empire in the 13th century. In fact, Serbia saw its glory years in the 14th century, going so far as to call itself an empire and the successor to Byzantium (even though the Byzantine Empire still existed). The glory was short-lived, though, for at the same time the Ottomans were growing in power, and eventually Serbia was absorbed into the Ottoman Empire.
Byzantine Empire
the Byzantine Empire, which by 1300 consisted of a few strips of land on the northern Turkish coast, plus a bit on the European mainland and some Aegean islands. The Empire had been in decline for a long time, with a low point reached when Christian crusaders captured Constantinople in 1205. While the Greeks came back to power in the 1250s, they never were able to recover the strength of the old empire. This was mainly because of the rise of the Ottomans in Anatolia. They would later, in 1453, finally destroy the old Empire, and Constantinople became Istanbul.