I have one recommendation, it is really special:
In town Smederevo 40 km. From Belgrade is manifestation called day of vine. Great program, costumed actors, fair, and the biggest flat fortress in Europe. There is free transport by boat, starting at 12:00h from port called - kej 25 majAll is for free.
For getting back is organized bus, at 10:30 PM , from the Smederevo port. It is free too.
If you like to much and you want to stay one ore more night I recommend Hotel Car for overnight (15 Euro I bed in double bed room). Bus for Belgrade is 4 Euro in one direction.
YOU SHOULD SEE THAT COUST FROM THE BOAT AND THAT FAIR, vine is excellent and very very cheap.
!!!!! The number that you can register for FREE promotional boat ride "Topčiderka" 7.Sept to Smederevo is 0648449326.
Number of places is limited!
Tell someone in hostel where you have accommodation to reserve places for you or call by yourself!
I translated :
As part of the opening of the 125th commercial and tourist events "Smederevo autumn", Friday, 7 September will be organized promotional FREE boat ride "Topčiderka" for guests, both foreign and domestic journalists, and representatives of tourist agencies from Belgrade to take a day trip who will start at 12:00 from the port called - 25. MAJ.
Arrival of boat on the Danube in Smederevo port is expected in 3:30 PM.
Afterwards, guests will have the opportunity to tour the city and attend the traditional ceremony parade.
At 5:30 PM Is planed tour of Wine, trade fair, village courtyards and agricultural exhibition in the fortress.
Guests and journalists who arrived by boat can attend at 8:00 PM. Opening of the manifestation "Smederevo autumn" is in the small town of Smederevo Fortress.
Return to Belgrade is organised by bus at 10:30 PM from the Danube port in Smederevo..
Hope you will enjoy