A minecart at the end of a range, a locked iron door and a chest full of bows and arrows. Shoot the minecart to open the door! When the minecart pops off after the arrow breaks it, it no longer powers the detector rail it was sitting on, which turned off a torch, but now the rail has nothing on it, the power turns off, the torch turns on, the door opens! Good for adventure maps or archery challenges. The distance can be as far as the bow can shoot, you could use multiple minecarts or a different thing to activate like a trapdoor or explosion.

I will finish this map off and post a download link. All my contraptions thus far have been in this map :)