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I seem to recall building small AM transmitters in elementary school. I don't remember if I used a transistor or not. I think it was easy to construct the capacitor and the inductor. (I build it in the early 70's and don't really remember how.)

We also made crystal diode radios, but I have no idea how to construct a diode from scratch, so maybe those are not simple.

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A pencil tip touching a steel razor blade is famously a diode which can be used to build a "foxhole" radio [1].

[1] https://en.wikipedia.org/wiki/Foxhole_radio

cat whisker detector

This one just requires a lump of germanium and a pin or small wire.

Traditionally, one would have used galena (natural lead II sulfide) crystals. I also recall most "boys'" radios - the type you'd find in Cubs/Scouts/etc. handbooks - using a razor blade as the contact whisker, probably because it was the simplest way to get a small contact point and stiffness at the same time with materials a kid would likely have to hand.

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