Higher Lower
Higher Lower is the epitome of a simple, no-setup, re-playable game that's a ton of fun. It's literally your classic, "I'm thinking of a number between 1 and 100" game with an awesome screen element that makes it even better. Have a student guess the number in your head before the timer runs out. All you have to do is hit the arrow keys on the keyboard to show if your number is higher or lower. It's a great game when you're in a pinch and didn't plan a game :)