Advice for the type of sensor to use

I want to make a puzzle where three items must be picked up in the correct order for a lock to unlock. I want to use various types of items ,for instance a letter, a key, and a pendant. I am hoping to hide the three sensors below a thin layer of wood so not to be noticed by the user.
My question is does anyone know a type of sensor that could accomplish this? I was thinking RFID would work but would require a tag and I would like to be able to use items without tags.

Maybe a photo transistor.

Could you attach a tiny rare earth magnet to each item to be detected with Hall effect switches?

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Yes, I thought of that but the magnet would be noticeable on something like a letter or piece of paper. I could make three small metal detector coils too. But once again it would work with paper.

I was thinking that also or maybe a photo diode. The only problem I see is that I would have to have a small hole in the wood above I've sensor. Hoping to stay away from that but might have to go that route.

Or simple reed switches.

If you want to be tricky and your items can be of metal or partly of metal you could:

  1. put a coil under each part of the puzzle
  2. Ping each coil with a pulse of current (Mosfet)
  3. Read the frequency (will change when the metal is removed.

Personally I would go with a small Neo magnet and a reed switch under the layer of wood. Simple and functional.