I don't have any RPG / Adventure games for the DS Lite, but I would highly recommend New Super Mario Bros. It's a very fun game and takes full advantage of the DS - controls, touch screen, and even the mic in one of the mini-games.
Quick plug too for Advance Wars: Dual Strike, which you should be able to snap up for around $10 I think (assuming you can find it new anymore). Best strategy game for the DS, and tons of replay value built-in.
Edit: sorry - just checked, nobody has AWDS new, and prices are well north of $30 on some sites.