I'll be there the 12th-25th. Went last year too. Love the city. I was quite surprised at how many parks there were. Kalemegdan was great. A lot of stuff going on around there. Tesla museum was sweet. Old Belgrade has a lot of great walking/exploring. If you are a soccer fan both Red Star and Partizan both have good teams and it's a very different atmosphere than MLS. Tickets are also super cheap. I stayed at the very very top of western city gate, you'll know it when you go into belgrade from the airport, which had an amazing view of the city. I would stop at Mandarina Cake Shop everyday on the way to the arena. They had delicious desserts, sandwiches, and breakfast.