A simple security camera ?

[dirtychinchilla]

I quite like the trip-wire log suggestion.

Until you walk in and forget to disable the trap!

 

[Willir]

Tripwire attached to large swinging log is my recommendation.

 

[subbytna]

Won't your SD card be filled with pictures of you then?

coffee went over the monitor at that reply. You win the internet for today

 

[Twirlywoos]

A job worth doing is worth doing.... Oh wait a minute.

 

[SimonCHere]

Just google for trail cameras.
exactly what you want and pretty cheap.