If anyone remembers UFO:Enemy Unknown (XCom) or even RebelStar/Laser Squad from back in the day, XCOM: Enemy Within on iPad has just dropped to $3, one of the all-time best games if you like strategy/tactical stuff.
My other favourite games (which I still play after years) are Carcassonne (board game) and FTL (space combat strategy game). All of these are great value as you’ll probably have only just figured out the basics even after a long flight!
My more recent-ish top pick is The Battle of Polytopia (formerly SuperTribes) which is a very quick playing, very light Civ style game (but played for high scores, old-school style!)