I keep vivariums at home and there are a number of things that I use timers for that I would much rather have an arduino controlling.
Humidity is currently controlled by a timed ultrasonic mister (you can modify a very cheap wallgreens ‘cool mist humidifier’ with a piece of pool hose to make a great ultrasonic mister for moss etc). I would prefer some sort of sensor to detect humidity and turn the mister on and off accordingly.
Temperature is a problem in my vivs. I have fans which I rarely use, even though my tanks often get too hot due to lights, because if I leave the fans on too long they dry out the tank. If I had momentary fans based on temperature readings, or some other temperature control (ice probe aquarium cooler in the water? I don’t know.) that would be great.
Lighting is currently a timer with several full spectrum plant lights during the day and a strip of blue led’s at night. It would be cool to have a MaxM or other powerful rgb LED simulate a gradual shift into the night phase in addition to the day lights.
Water level: the water feature evaporates quickly due to the lights. It would be nice to have this monitored and regulated by an arduino controlled pump and reservoir.