I'm a big fan of the Kosmos 2-player seriesKosmos 2-player series; not all of the games are great, but most of them are solid, and they're all geared specifically for two players, they come in small and easily portable boxes, and they're generally reasonably light while having just enough strategic depth to keep them interesting.
My favorite is Kahuna, as I find the gameplay a bit more strategically interesting than most, and my girlfriend likes Odin's Ravens best. I've heard good things about Lost Cities, but never had a chance to play it. There are many good games in the series; you can try sorting them by rank on BoardGameGeek, and look through the first few pages to find the best ones.