As an Erasmus student you can pick single courses from every field of interest, and you don't have to decide until a few weeks after start of studies. Use those weeks, visit as many classes as possible, talk to the lectures, before you decide which courses you really want to take. But timetables change a lot in the first two months, so your carefully selected personal timetable might just collaps like a house of cards and you'll suddenly have overlapping courses or long breaks between classes.