You can choose to take courses that are not part of your own degree programme and that therefore do not count towards your diploma. These courses are not in your curriculum and are therefore 'extracurricular'.
You can also apply for the Honours Programme. Through this programme you will follow 30 EC in extracurricular courses. You receive an Honours mentioning on your diploma if you complete the Honours Programme as well as your bachelor's programme within three years, with a grade average of at least 7.5 for your regular bachelor courses.
If you want to include an extracurricular course in your degree programme, you can submit a request to the Examination Board of your programme. See how to do this on Submit externally achieved result.