Stay in contact with your tutor about your thesis, enrolment in courses once you are back, and other relevant matters. Although you may not be able to meet with your tutor in person for mid-term evaluations, you will still need to fill out the mid-term evaluation form.
Be sure that you register at the Dutch embassy or consulate abroad. You do not have to deregister from the Municipality of Utrecht if you are away for less than 8 months.
Make sure your mail is forwarded or checked by someone. It is also recommendable to authorise someone (e.g. a parent) to represent you in case of student finance or other bank issues. Be sure to keep checking your UU Gmail regularly.