I would just mention that formal volunteering is still not very well-regulated by law in Serbia - you may not be able to work "officially" as a volunteer and get your contributions covered, and be able to use it as grounds for a visa application etc. So that is something to look into.
However, there are lots of opportunities to volunteer. If you are in Vojvodina, try the Ecumenical Humanitarian Organisation, who do a lot of good work in this area. If you want to help Syrian refugees in the north of Serbia you can track down Zoltan Nyul in Stara Moravica, who runs a humanitarian charity and is working 7 days a week getting basic help to people in transit through Serbia.
