Doing puzzles regularly can help keep your mind sharp. Puzzles come in all shapes and sizes, from very simple "connect the dots with the fewest number of lines" brain teasers done with pencils and paper, to incredibly complex wooden boxes with secret mechanisms, to nearly impenetrable codes like the ones on the Kryptos sculpture.
On the "simple" end is this weaver's puzzle by Mike Reilly... simple to make, but so devilishly difficult to solve that you'd better take notes on what you did while you're making it, or even you might not be able to find the solution a second time.
Essentially, you have a small wooden board and some nails (or pins, tacks, pegs, etc). Nails are hammered into the board in a random pattern. A thread is attached to one of the nails, and on the board, there is an empty hole where a nail should be. That last nail is attached to the free end of the thread.
Your job, as a puzzle solver, is to wrap the thread around each peg only once, then place the free nail into its hole, without any excess string hanging off of the puzzle. It's appropriate for supervised kids, but with at least hundreds of possible solutions, it's not as easy as it sounds.
Since letting us know about his work, Mike has created a D.I.Y puzzle projects page. If you like the Weaver's Puzzle, the L.I.P.S. (Lines in Particular Shapes) Puzzle is definitely worth a look. It requires just common office supplies like graph paper, a straight edge, and a pen. What will he think of next?