For a centerpiece—how about a stack of mystery novels for kids and other books that amateur detectives might need? Flank the stack with the two small jars of chocolate kisses that are part of the guessing game.
If you have a piece of tartan cloth or wish to acquire some for a tablecloth or a runner, it will make a colorful, fun accent for your table décor. Toy magnifying glasses are inexpensive. As are penlights. They both look fun on a mystery party table and make good party favors that your guests can take home.
A placemat can be made by taping together printouts (use cardstock) of Word Search and Edinburgh Sudoku.