Here is a link to a tutorial on how to make a very effective, fairly simple and undetectable (to the naked eye at least) pitfall trap. You could easily disguise this into a mineshaft or other form of tunnel, and you even get to keep the items. Unfortunately there is no reasonable way for you to get past it yourself without setting it off, however when I built this to protect my house this was just a decoy entrance, and the actual entrance was off to the side at the bottom of a small lake.

http://www.youtube.com/watch?v=Qpt8-JMP0gA

Enjoy :)