I like the claymore idea but im sure you can get tripflares comercially. I think this might be a good idea as well.

http://www.youtube.com/watch?v=4D_3nAfmcqY&NR=1

It must be available in the USA because they sell them over here in a health and safety conscious Britain.

If you think of it then its a fairly simple bit of kit to fabricate as well.

I would imagine if you used that with a small electrical switch as well you could put an electric light on to go on at the same time.

Before the war on terror, if I saw an unattended package I used to think "I'll be having that!"