@freemo An actual calendar that I put misleading information on, a small notebook with the information mixed in with nonsensical rambling and pictures of things. It takes all three to put it together.
I'll set alarms on my smartphone at the wrong time and label them the wrong thing.
For you, I'd recommend a picture and NFC tag that stores a key. You just embed a photo with a bunch of encrypted information and use a Yubikey to decrypt it. You can do the same with Wav files and then embed that. The NFC tag should just link to a website that lets you know who touched it.