This is an easy job if you plan well.
Here are some tips for getting a word:
You can pick a word from the dictionary.
Make note of any interesting words you hear or read.
The Word of the Day at Oxford Dictionaries or Merriam-Webster.
The English Language & Usage forum can also be a source of interesting words.
Write the picked word on the whiteboard:
The word and its definition in large so they can be read from a distance.
Remember that there is a twenty five cent find for anyone who speaks and does not use the word.