Study association GEWIS
GEWIS, the study association of the Department of Mathematics and Computer Science is there for you as a student. The association provides the department's introductory week and organizes conferences, sports tournaments and excursions (to far-flung international destinations). You can also purchase books cheaply through the association. It therefore provides you with a multitude of benefits and you may well make friends for life.
The students in GEWIS are also involved in developing the course program and in the further day-to-day matters of the department. The association has its own room where you can drop in at lunchtimes and during the weekly social, whether to socialize or to discuss more serious matters.
More information at the website of GEWIS.