In 2008, I first got Brain Training sometime after I got my Nintendo DS Lite. Brain Training is an edutainment game which requires you to "train" your brain by completing various tasks like counting how many people are in a house, quickfire maths and playing sudoku. While this game doesn't actually train your brain, the game has some nice and challenging mini-games which will occupy your time.
I find this a really good way to kill some time after a long day or you want to practise for a test you may be taking. There are a few modes which include Brain Check, Download Play, Sudoku, Brain Age Check, Puzzles and Quick Play. For every day you put a stamp on the calendar, you will unlock a new mini-game. The main character in this game is Dr Kawashima, who will help you with the tasks while providing some knowledge and humorous commentary.
While the game doesn't do what it says on the box and the game may not be for everyone, this is a casual game which is meant for people who need a casual game to play in their spare time.