Maybe you have some challenging puzzles, strategy games, books with logic puzzles, and other fun-yet-educational odds and ends. Consider putting them all in one place and giving the space a name...Brain Gym, The Puzzle Place, Conundrum Corner - you get the idea.
By making a Puzzle Place (or whatever you want to call it), you make materials that may have been hidden in a closet, inviting and accessible to students. Those kids who tend to finish everything early will love a center that offers them new challenges. It will also become a popular choice during free time and indoor recess.
There are all kinds of thing you can put in your Puzzle Place. Here are some ideas:
Puzzles: The trick with puzzles is to keep all the pieces together. Choose puzzles without too many components and be sure each puzzle has a home of its own. It is also good to choose puzzles that are quick to put away - you solve it or you don't in the limited time you have, and then it goes back to its home...no saving allowed. Some ideas include Tangrams, Happy Cubes, Soma Cube, Brick By Brick, Rush Hour, Toothpick Puzzles, just to name a few.
Pencil & Paper Puzzles I am particularly fond of Grid Logic Puzzles because they work on so many levels. But there is also Sudoku, and tons and tons of brain teasers and other puzzles. I am also quite keen on Evan Moor's Critical & Creative Thinking Activities series, but that makes sense since I wrote them. Another option is the Highlights magazine, Puzzlemania, you can pick those up for almost nothing at yard sales. Get out your favorite book, copy some off and put them in your Puzzle Place. Rotate frequently.
Games Like puzzles, choose ones without tons of pieces and that don't take forever to play. Some of my favorites for the classroom include: SET, Blokus 3D (I love Classic Bokus, but too many tiny pieces for the classroom), Mancala, Boggle, Connect 4, and of course chess.
Be sure to check out the rest of the Creative Classroom Series.