Forget Sudoku. Kakuro: get your koat. Picross is the logic-based mind-meddler of champions. It's well established in Japan - having been invented by a woman flicking a skyscraper's lights on and off, believe it or not. Now the infection's spread to DS.
It works like PC timewaster Minesweeper with the randomness taken out. Starting out with a totally blank grid, you chisel out squares with your stylus, according to the numbers at the edges. So: seeing "3" and "7" at the top of a column means that, somewhere in that column, there's a line of three squares and a line of seven - in that order. The only question is: where?
Sudoku-heads used to just cross-referencing rows and columns had better upgrade their brain batteries now. Picross has no clues (unless you choose to autofill a single row and column at the start - wimp), so you really have to tease out the pattern, using knowledge of where squares definitely (and definitely cannot) lie. It's intoxicatingly logical and hard work. Imagine slowly illuminating a lightbulb with concentrated thought and you'll get a feel for how satisfying it is to watch the grid gradually fill up - blossom, really - as your beleaguered brain spots the rules and patterns. And - bonus - you've completed a nice picture of, say, a yawning hippo at the end of it all.