Alright just got a stupid idea, here me out.
A digital guest book. QR code in the toilet to scan, if you are in the WiFi network a website opens up.
That website does list all entries done in the past as a readable list. Also you can click on a "+" icon to add one on your own. After entering a name and a message it's added to that list. The entry will save that together with the IP and date time to restrict entries to every 24h per IP-address.

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