A while ago I bought an e-paper and displayed some images with my RasperryPi on it. See here for a video of it, with a link to full source code etc. in the description:

The amazing thing about e-papers is that the image lasts without power forever (I tested it for months), and the contrast is very good. So the idea is to build a small device with e-paper and solar cells, and then you can write or draw on the e-paper with your smartphone (maybe over BLE), or even remotely from anywhere over the internet to show a message (probably with WiFi, or a BLE relay). The case will have a magnet to stick it on a fridge. Of course, will use a low-power microcontroller instead of an RPi and some intelligent low-power polling mechanism.

This is a complete new dimension of the usual sticky notes on fridges, and it saves trees, too!