Eko-centre Caput Insulae protects Eurasian Griffonds and looks for volunteers to take part in projects on the island of Cres. More info here.
ISV - International Student Volunteers offers various programs for Croatia:
Volunteers For Peace offers over 3000 affordable, short-term voluntary service projects in 100+ countries, including a small number in Croatia each summer. More info here.
Bottlenose Dolphin Research Project. More info here.