We have a 2.5 oz magnetic version of Scrabble that we'll take now and then. We only use it on short mileage days or when it's rainy, though, so we've learned to leave it behind on some trips. That said, it's about as much game as you could possibly want for the weight!
A more recent and exciting find was the game Cosmic Wimpout. My mom bought it for us for Christmas because she thought it'd be great for backpacking. I haven't weighed it (sorry!), but it can't be more than 1.5 oz. It consists of 5 small dice in a plastic tube and is a ton of fun. We haven't taken it backpacking, but it should be great because it's easy, light, and there are no small pieces to lose (unlike the magnetic travel scrabble.)
Aside from that, sometimes we'll play some mental games like six-degrees of separation with movie stars, 20 questions, etc.