We've been playing lots of Tichu when we have 4 players around. It is a card game similar to spades or hearts, but a bit more fun. Card games are great because they don't take up any room.
RoboRally takes up a bit more space, but it is pretty fun even with only 2 players and that is ideal. You run around a board with little robot pieces that you have to "program" by selecting 5 moves ahead of time. You miscalculate a lot and run into each other and accidentally kill each others robots. Pretty fun really.
Jenny and I have always been fans of Scrabble, and we have a travel-Scrabble version that is very boat conducive. The letters clip onto the board and it all folds down in a neat package. Perfect!
One of Jenny's favorites has become Munchkin. In particular, Munchkin Booty. It is a card game, which is great on being a space saver, but it takes more than 2 people to make it worth while and that doesn't always work out as well. Your character is a little munchkin and you collect cards that help you fight monsters and you can plot with each other and whatnot. Neat.
Mark has a ton of games on his boat (single guy on a Catalina 42--he has tons of room) So we've also been playing Ra, Carcassone, Settlers of Catan, Dominion, Puerto Rico, and others, but they are mostly all multiple player games. (Dominion is fun and can be 2 player, tons and tons of cards though and it's more fun with 3+ players) If anyone has more ideas on some fun two player games, let us know. Thanks. :)
Oh, and we need to learn Cribbage...