in the context given it means he suddenly changed his mind he was sailing in the Caribbean and then suddenly he decides to go hiking in the German countryside

"...one minute...and the next..." means that the situation changes suddenly. Here are some examples from the dictionary: Things kept changing from one minute to the next. One minute they’re madly in love and the next they’ve split up again. Jobs are there one minute, gone the next. One minute you're happy, and the next you feel sad.

ONE MINUTE in this case means AT ONE POINT IN TIME. I don't understand the context, but this implies that he instantly goes from sailing in the carribean to hiking the german countryside.