Wiring a 128x64 OLED display via the I2C protocol to Arduino, and the Adafruit GFX and SSD1306 libraries used.
Wiring the BME280 sensor, which adds humidity measurement to pressure and temperature, and the difference from the BMP180.
An explanation of how a water level sensor works via alternating copper traces that conduct when touched by water, and wiring it to Arduino.
An explanation of how a rain sensor works internally as a variable resistor and LM393 circuit, and details on wiring it to Arduino with a lifespan note.