There are old-fashioned ones that suit this need just fine, like Tic-Tac-Toe or the Word Search puzzles that one of my aunts used to love. Nowadays, I just turn to my smartphone for some electronic exercises to while away any leisure seconds that I have.
Classics like Tetris, Minesweeper, or Mahjonng are as entertaining as ever. There are also free apps of 3D Bowling, Solitaire, Sudoku, Paper Toss, and Words with Friends that are still a delightful waste of time that also let the brainwaves relax a bit while still massaging the cerebrum (or is it the cerebellum?)
Some of my favorites are Dragon Fly in which you swoop through hilly landscapes as a baby dragon, collecting gems; Rollerball, a variation of the carnival classic Skee Ball in which you can win and collect some pretty cool virtual prizes the more points you score; and Teeter, a simple but challenging game in which you move your phone to roll a marble through a little maze in an attempt to zip it to its destination.
Looking for more? Try Jewels Deluxe, or Marble Blast, or the entire Angry Birds franchise. Don't get too addicted to them, though. These games are a great way to let loose, but eventually, real work beckons to attack the frazzled mind again.