So, an simple but hard to notice trap is to chop down a tree, dig a 1x1 hole 2 blocks down, place 1 block of tnt at the bottom, (if you want to do more tnt dig out sideways where the tnt is supposed to go then fill that with tnt.) then put a pressure plate on the tnt and build a tree on top of it.
you can also try using scaffolding to create a very tall tower then making an bowl shaped structure at he top then filling it with sand then lava.
thirdly, you can make a plain with the same method described in the first prank, make the players want to mine the grass ( for example put a bunch of cats underneath the tnt to make a strange sound)