Students in my class love completing the Dictionary task - Word of the Week. I laminate poster and put it up on display. I use a whiteboard marker to write a new word in the space every week. It improves their understanding of unfamiliar words and gives them the chance to learn new ones that can be applied to their writing. Their alphabetical/ reference skills also develop quickly as they become more competent at completing this task.
See also my THESAURUS/ SYNONYMS POSTER, which can be used in a similar way. Both help to increase their range of imaginative and adventurous vocabulary.