Digital thermostat

The simplest case possible, but surprisingly useful.

digital temperature measurement
GPIO control of a relay switch (just plain LOW/HIGH control but plug it into a relay)
logic on the device to open / close the relay depending on temperature conditions, with some hysteresis

Voltage logger for car battery

Maybe you have a machine / car / forklift etc that often fails in the winter? You can make the following:

measure the battery level with an ADC input (don’t forget about a proper voltage divider!)
log that data over WiFi & mqtt
set up some cloud based online alerts about low voltage level / reminder to charge the device

You can use a joystick plugged in to two analog ports to manipulate two-axis of freedom via two servos.

You can simply automate some processes based on time:

either using network time protocol with an ESP8266 (NTPClient library)
or using a hardware real time clock module attached over I2C/SPI And then setup some IO changes at specific times, move some motors/servos etc.

You could set up plant soil monitoring for low humidity and set up some alerts either over WiFi or a buzzer / LED.

You can work in pairs / groups to build compliant devices - a temperature sensor and a remote display.