Here's an activity that incorporates vocabulary, spelling, communication, and dictionary use. I thought of it about 10 minutes before class one day, and refined it for a different class later in the day.
Start off the class by asking if anyone can stump you with a word that you don't know. Do this for about five minutes and then talk about what ways you can build your vocabulary and where you can find the definitions of words.
Next, give the class about 5-10 minutes to write down a few really good vocabulary words as well as the part of speech and the definition. You may also want them to use the word in a sentence.
Break the class into 2-4 teams depending on class size and collect the word lists.
Have two people go to the board from each team, with one of them standing at the board waiting to write, and the other sitting in a desk facing away from the board with just a dictionary.
Tell them that you will say a word from one of the lists. The person at the board must write the word and write the definition of the word. The word must be spelled correctly and the definition must be correct. When they are done, they put the marker down. First team to have both the correct spelling and a correct definition earns a point.
The catch is the person sitting in the desk can't look at the board and the person writing at the board can't look at the dictionary. The two of them have to work together in order to figure out words they don't know the spelling or definition of.
After the round is over, the pair switch places. After the second round, a new set of people go to the board and the process starts all over again.
The students quickly figure a few things: It pays to make sure the word is spelled correctly because once you put your marker down it's done. It also pays to write as brief a definition that still makes sense. It's a good lesson in brevity.
Some words of caution: Depending on what type of atmosphere you like in your class, you may want to discourage the students who are not at the board from yelling out things. Of course, this could also be helpful and add a different dimension.
Overall, this is a fun activity that the students tend to enjoy and can really get into. A lot of times the students who are sitting down will be looking up new words to stump the people at the board. Also, a strong-weak pair can still do just as well against a strong-strong pair if they can work together.