Many smaller NGOs working in Greece are in need of volunteers.
Certain skills are highly sought after; such as legal training, or experience teaching or in logistics. However, anyone committed and motivated to help will undoubtedly be an asset, and training is always provided.
Fitting in a longer stretch of volunteer work is not possible for everyone. However, there might be organisations who need a once weekly, or monthly volunteer with your skills. For example, to help maintain a website or run social media campaigns remotely.