Good site to search: http://www.holland.com/uk/search/groepsaccommodatie.jsp
Possible places, please comment:
Website only in dutch, 8 sleeping rooms for 2-6 persons, 36 beds in total, kitchen, close to lake
Public transport seems available via (cheap) taxi bus and train to Leiden
Added on Mon, Jan 12th:
Kitchen and large living room available, 24 or 32 beds
Kitchen and large rooms, 28 beds, at the coast! Availability limited to midweeks?