A very simple trap that is virtually undetectable is the "landmine" you have carpet covering redstone ore and an observer observing the redstone ore. Then you hook the observer to a bunch of tnt and when the player walks over the ore (covered with carpet) it will emit particles which counts as a block change so the observer activates the tnt and blows the player to smithereens. (just make sure when you are setting it up to wait until after the particles are gone before you add the tnt because the carpet being placed on the ore will cause the ore to emit particles)

A very simple trap that is virtually undetectable is the "landmine" you have carpet covering redstone ore and an observer observing the redstone ore. Then you hook the observer to a bunch of tnt and when the player walks over the ore (covered with carpet) it will emit particles which counts as a block change so the observer activates the tnt and blows the player to smithereens. (just make sure when you are setting it up to wait until after the particles are gone before you add the tnt because the carpet being placed on the ore will cause the ore to emit particles) Heres a screenshot:

(Obviously you don't need that much tnt)