Voluntary work is something that you consider doing as a means of updating your skills, or as a constructive way of using your time and contributing to/being a part of the community.

ACCESS – this organisation relies on English-speaking international and Dutch volunteers who enjoy helping others adjust to life in the Netherlands.
Contrast Leiden - a student based organisation connecting international and Dutch students.
Charity - a general search platform for volunteering opportunities in the Leiden region.
DUWO – an organisation that offers discount rental housing to students in exchange for volunteering work.
Home in Leiden - a web based information provider to international families in Leiden.
LIBC - a brain and cognition research institute looking for volunteers to take part in research projects.
Meetup – a platform for social groups and clubs offering various meet-up activities for expats, sometimes including volunteering.
Vrijplaats – a local organisation with various projects that offer possibilities for volunteering.

You may also be able to volunteer for international schools as a reading/library assistant or lunch time supervisor for example. If your Dutch is sufficient it may be possible to volunteer in local schools, foodbanks and charity shops etc..