You have received the accessory kit for your Gamebuino, and you do not know what to do with it?
Start by following the introductory tutorials, and the ideas will come on their own 
Accessory kit sheets
Moisture sensor : to watch over your plants.
Joystick : to develop great games
Rotary potentiometer : it turns !
Ultrasonic detector HC-SR04 : to measure distances like a pro down to the centimeter.
Servomotor SG90 9g : give your META an arm.