My current employment terminates on 31st December. I understand I will have to give back the work permit but what about the Non B visa? Does this also have to be cancelled or could I use if for let's say another 1 or 2 months?

Moved to visa forum.

The work permit needs to be cancelled.

What happens to your "visa", it actually a permisison to stay depends on your situation. If you have a valid non-B visa and have to leave the coutnry every 90 days yur permisison to stay and visa remain valid. If on the otherhand you have an extension of stay from immigration and do 90 day reports, you have to cancel your permision to stay at immigration and leave the country. Immigration can give you an additional 7 days for 1,900 baht.