I see you've already gotten a lot of great suggestions, but I'll add two more that I haven't seen yet:

• Spirit Island: A fantastic game in which you play a nature spirit defending their island from the invading settlers. Great opportunities for tactical decisions, with very little luck involved. All the different spirits you can play are unique, and make each game entirely different.

• Sprawlopolis: A Button Shy game, this game consists of 18 cards only (it fits in your pocket). All cards have city blocks, parks, and roads on them. You put the cards down one by one, adjacent or (partially) on top of others, and build your city. Each card has scoring conditions on its back, and every game you pick 3, so you always have different goals for winning. Pretty challenging game! It's quick to play and due to its size you can bring it anywhere with you.