I am doing volunteer work in Germany, and I want to go to Croatia on summer vacations. I have a Schengen visa. Is that enough for me to visit Croatia?
It depends on the type of Schengen visa you have: a two-entry or multiple-entry visa would exempt you from the visa requirement in Croatia, a single-entry visa does not.
If you were a resident in the Schengen area (holding a national long-stay or “type D“ visa or a residence permit), you would also be exempted from the visa requirement in Croatia.
More details can be found on the website of the Croatian ministry for foreign affairs.
Looking at https://en.wikipedia.org/wiki/Schengen_Area - Croatia is not currently in the Schengen area, so you would need to apply for a visa in order to visit.
The good news is that they are working towards membership.