The Cactus Trap

Step 1: Dig a fairly deep hole (can be any width you want, depending on how many cacti you have) which goes down.

Step 2: Make sure one side of the hole has a in one block higher than the rest (meaning the edge - at the top - of the hole). This is so the victim won't see the hole. If this hole is for mobs, ignore this step.

Step 3: fill the bottom of the hole with cacti.

Step 4: Now just wait until the victim falls to his/her death!

This cactus trap is a simple and easy/quick to make version. Check out Youtube or whatever to find better cactus traps.