Samo sam na proputovanju means “I am just passing through” – the longer version of the previous phrase.
Your group gathered on Saturday morning in a village near the border to think about the best route through Croatia: Saturday (Samstag in German), gathered and thinking give you samo sam na. Then you toasted each other with tomato juice, spilled some on somebody’s pullover – and that person suddenly looks incredibly young. Toasting, pullover, tomato juice and young give you proputovanju, with the stress on the tomato juice.
As you repeat it, pay attention to where the stress falls: in the image, the stressed part is always the most conspicuous one.