Children of Mana (for Nintendo DS) is easily one of the most enjoyable RPG game I've played after a long time. The last rpg game that I had played was Suikoden 5 for PS2. Children of Mana is a very simple rpg game with very limited rpg features. It has a very basic set of weaponary and character upgradation system. This is easily a very pick-up and play kind of rpg game. Though it was criticised for this very easy aand not very in-depth rpg element gameplay, but I really enjoyed the game. I think the DS is made to enjoy a very casual and easy pick-em-up and play kind of games. Especially RPG games can be very engrossing and might require a lot of attension and dedication to play through the entire game. Children of Mana easily accomplishes this by having the correct balance of rpg elements and casual appeal.
Some links about this game:
http://www.seikens.com/children-of-mana.html
http://en.wikipedia.org/wiki/Children_of_Mana