[BastiS]

This is one I was thinking to try myself... let me know if you like the idea.

Get into a secluded woodland area - you should know your ways around. You'll need a smartphone with a dataplan, use KIK or something similar.
Temperatures should be high enough to not catch a cold from running around naked.

Find a hiding spot, get naked (I recommend to keep sneakers on), hide your clothes well (you might be away for a while).
Now what? Get a guide!
Maybe you know someone who should be your guide or get a volunteer (shouldn't be to hard to find here, should it? I would)

Now this goes like a classic pen&paper roll playing game. Tell your guide what you see and ask what to do. If you got a fair data connection use voice (much faster than typing out messages especially if you also want to concentrate on your surrondings to not bump into someone taking a walk.
Examples:
You follow a small path and get to a fork - send your guid a pic and ask where to continue.
You hear voices nearby - ask if you should try to hide and wait, sneak closer to see whats going on, try to get away quickly without beeing seen
You find a raised blind - ask if you should explore / get inside, or go somewhere else...

Plan how long this should go. And when to start heading back. Mark your starting place GPS location on a map on your phone to be sure to find your way back. Make sure the battery is full before you start.

If you want to take this a step further - add a tracking software on your mobile and allow your guide to see where exactly you are on a google map or similar - this would allow your guide to keep you in a more save(-ish) area, guide you to interesting, exciting, risky locations and also guide you back to your startin point (this would of course require a very high level of trust between the player and the guide)