If you're looking for a challenge but don't feel like spending a lot, then Picross DS is the only way to go.
Picross is a combination of Pictures and Crosswords. You are given a grid (eg. 10x10 of 15x15) and a series of numbers for each row and column. Your job is to take the numbers they give you, fill the correct amount of grids to eventually draw an image. The numbers might be something simple; in a 15x15 grid, if a row has 15 beside it, then you know that all fifteen blocks will need to be shaded. Other times, you'll get a couple of numbers (eg. 5 1 3 1) where you need to fill in five blocks, brake for at least one, fill in one, brake for at least one, fill in 3, break for at least one, fill in one and break for at at least one (or the rest of the board). Depending on the size of the board, it might be pretty simple to figure out, but the more challenging puzzles will require you to work both rows and columns in intervals to figure the puzzles.
The game works well to introduce you to the processes involved in the game. Similar to Sudoku in Brain Age, you get penalized for incorrect answers, which hurts your time. Since you're given an hour to complete each puzzle, it's better to try to eliminate blocks before touching them in certainty that it is correct. Completing the puzzle in less than the hour limit, gives you a small little reward with the image. If you complete it after the hour, you'll want to go back and try again to get the mini prize, although, the outcome doesn't really change the game in any way.
There are numerous puzzles and you can even go online and download more. Not to mention, you can create your own puzzles to trade with friends.
With most 'Touch Generation' DS games, this one is fun for everyone. You'll want to have your friends try it out to see if they can beat your time and you, yourself will want to improve on your own times. The difficulty might put some people off, but like any good game, the challenge is necessary to truly enjoy games like this. Also, at 19.99 (24.99 CDN), you really are getting more than you paid for.
The DS is known for it's great pick-up-and-play titles and Picross DS is just another title that fits the mold well. Don't get upset if you're finding yourself stuck on certain puzzles; the end result, once you've completed, is that you'll feel that you've actually accomplished something and you might think that you're actually smarter than you thought.