Well, my favorites are: * DS18B20 - One-wire temperature sensor that is cheap * Generic PIR (motion) sensors - you can find simple motion sensor boards for ch

Well, my favorites are: * DS18B20 - One-wire temperature sensor that is cheap * Generic PIR (motion) sensors - you can find simple motion sensor boards for cheap and they are pretty fun. * Capacitive sensors - you can make a touch sensor/button out of anything conductive that only uses a single wire (not even ground, just one wire) * Ultrasonic distance detectors * Hall effect sensors - detect presence of a magnet, especially useful with stepper motors * BME280 - temperature/barometric pressure/humidity, one of the best all around sensors for weather monitoring as it's all on one chip. Also supports both i2c and SPI. Note it's BME280, the BMP280 doesn't support humidity. A good bet is to buy a kit that gives you a random bunch of sensors to play with. You can find them cheap.