All Bachelor's programmes offer 15-30 ECTS space for minor programmes. The first semester of the third year is designated as a minor and exchange semester. You can, for instance, follow the Education minor or the minor in computing science during that semester. Please note, however, that many minor programmes are set up in a way that requires you to take courses over the course of a year or more. Therefore, it is worthwhile to think about which minor programme you might want to do early on.
You may also put together a series of related courses (e.g. on economy, or on nano science, or on mathematics). You must get approval from the examination board to incorporate this minor in your programme.
Doing a minor programme reduces the number of elective courses you will have to take. It will also give you the possibility of looking beyond your own field. Some information on minor programmes can be found in the minor prospectus.