National Institute of Environmental Health Sciences has a terrific brainteaser, puzzle, and riddle page for kids and adults. There is plenty to choose from to work your mind in a fun way. One important way to build vocabulary knowledge in children is to teach them how to have fun with words (word consciousness) and this page is a step in the right direction.
Here is one of my favorite riddles from this site: The more of them you take, the more you leave behind. What are they?