This was originally planned to be a Christmas present for my parents, but because I got the idea so late, I realized I couldn't finish it by today. I've put it on hold so I can focus on my Trinket EDC Contest entries.
The shower has a textured glass panel next to the door. The clock will be in a Hammond enclosure mounted on the outside of the glass. That way, it doesn't need to be waterproof, though it still needs to be humidity-proof.
The display will be two 7-segment digits, made by myself because none easily available were the right size. It will alternate between the current hour and the current minute (with a colon on each side to indicate which). When you wave your hand over it, it will tell you how long you've been in the shower. Another wave will display the day of the week and the day of the month. Waves will be detected using the same light sensor used to turn it on and off automatically—whenever the shower lights are on, so is the clock.
Hammond 1591 XXD enclosure
the largest flanged enclosure available at my local electronics store
Arduino Pro Mini
328 – 3 V / 12 MHz
to make the display
Seeed Studio Grove RTC (DS1307) because that's all my local electronics store had
I was planning to use LEDs, but I think I'll switch to phototransistors.