Budget - 49 euros per day.
Belgrade is situated on two rivers - the Sava and the Danube. The medieval fortress of Kalemegdan is the heart of the city, it lies at the confluence of the Sava and Danube rivers. Near the fortress you can visit Stari Grad - the oldest part of Belgrade. There are a huge number of museums, expensive cafes and restaurants.
You should also visit the old quarter Skadarlija, Ada Ciganlija park, National Museum with an extensive archaeological collection and the collection of paintings, Old Palace, Ethnographic Museum with a collection of Serbian costumes and folk art, the Gallery of frescoes, the Residence of Princess Ljubica, which made in Balkan style with a large collection of furniture of the 19th century.
You will not be bored in this city!