I had a fat DS and now have the crimson DS lite. It's awesome, well worth the investment. There's a good selection of games, but anyway I'd recommend screen protectors (at least for the touch screen) and a case.
In terms of games, Professor Layton is really fun if you like puzzles. I'm playing Rune Factory 2 now, also fun if you like HM type games. The World Ends With You, Ninjatown, Cooking Mama are all good. Rhythm Heaven should be good too. I love RPGs, and the DS certainly had a lot of those.