5 Low quality metal
you put a metal tube on the wall with an open top and bottom, and put a grenade inside. The grenade is held by its pin to a trip wire( made of cloth, or you could use more low quality metal to make a real wire). so when someone trips the wire, it pulls the pin, when the pin is pulled the grenade drops down. Boom instant trap.
It will be visible so it will be tough to hide and make it anti fun. And say it con only be placed on walls or tree to prevent greifing.