[DS] Must Own Games for the DS

Tossing out a recommendation for Elite Beat Agents and/or Osu! Tatakae! Ouendan (and its sequel). If you hate rhythm/music games avoid like the devil, but otherwise check them out. Each round only lasts as long as the song, so you can play for as little or as long as you want. Another touchscreen-heavy game to try out would be Trauma Center. It doesn't take all that long to progress through each individual stage, but it may be difficult to return to if you keep putting it down (on account of difficulty going way up).

The king of short-but-addictive games is Puzzle Quest. It's Bejeweled with an RPG flair, and once you start it you instantly get sucked into the monotony of racking up mana and blowing up skulls.

Backing up the recommendation for Phoenix Wright / Ace Attorney games. They're actually one of the main reasons I bought a DS, and although I have a flash cart I actually own all 3 of the games released here (along with Mario Kart DS, which I bought while waiting for the flash cart to arrive). They're like an interactive book, so it's easy to start and stop it at your leisure. Plus you can save at nearly any point so you'll never have to lose data because you're in a hurry. If you like the series, you should also check out Hotel Dusk: Room 215.

Jam it back in, in the dark.