Smallworld, trust me on this one, the rules are dead simple, but is an absolute blast, 2 players would be under an hour to finish, and between the somewhat random nature of what you get keeps each game different and requiring you to switch up your strategy. It's probably one of my current favorite small group games right now.
Ra - The Dice Game is another good one, games pretty small making it easier to transport, and once again is super simple to understand.
Those are two on the top of my head, if you're looking for more I could probably throw out a few more good choices. Warning, I am not responsible for any breakup that might happen through the cut throat nature of board games.