Drawing from my own life...
* Homemade stroboscope using bright white led's. With a microcontroller you can build in several useful functions: calibration, smooth speed adjustment at all ranges, brightness, 'ON' time. (I made a very simple stroboscope which lacks these functions, but it is fun nevertheless.)
* Ceiling fan speed indicator. Mine has 3 speeds but no indicator. My idea is to mount a photosensor near the blades. Count time for 4 waveforms (or whatever is quantity of blades). Calculate rpm. Light up 1 or 2 or 3 led's to indicate speed.
* House voltage monitor. Handy for reading AC volt level. Add an audible alert if voltage drops below a certain level, suddenly or gradually, or blacks out. A large capacitor will power it for a few seconds when blackout hits.
* Stove/ range/ oven reminder. For those who tend to forget they left something cooking. Make a circuit to detect the range light is on, and sound a beeper every 60 seconds, loud enough to hear down the hall. Or a canned voice saying 'stove is on.'