I don't have a passport. I do have a Finnish identity card. It is a valid travel document across the EU. I have used it on planes to Berlin and London.
However, Croatia is not a EU member, nor part of any treaty that would make an identity card a valid travel document. However, the Croatian ministry of interior has, as far as I can read from their page: http://www.mup.hr/1839.aspx, decided that identity card IS a valid travel document there.
Has anyone tested this? I would prefer not getting a passport, as it is rather costly.