I'm planning a new geoache and the idea for the final stash is to place an ammobox with a lock. This way I can make sure that not all visitors who find the box, will be able to open it. In fact, if you don't know how to achieve the key, you won't be able to reach the logbook in the box.
Next to the hiding place, a birdhouse will hide the key. It will take some effort to make this failsafe, but that will not be the main issue. The key will be released by some intelligent circuitry that understands your "secret knock". I'm thinking something like: knock x times, wait a bit and then y times. With x and y being some numbers of the last few waypoints.
The main issue is making the motor failsafe, and this is my first effort. I think the result is great, it's strong and fast enough to let the key come out of the birdhouse fast enough (12 seconds down, 17 seconds up, the 'up' speed needs more torque and has a lower pulses per second).
I'm using a bipolar stepper motor with reduction gears. The 24 volts will be supplied by a DC-DC convertor that boosts one or two 18650 batteries (so 3.7V or 7.2V) to 24 volts. I tried one model and it works great. I can easily draw 2A current out of one battery, or 1A out of 2 batteries to make a strong enough 24V power supply that should last for 2 to 4 hours 'motor movement'.
I made this construction with a 10cm shaft holding a spindle that winds up a small chain with a key attached at the end. The shaft can rotate freely due to the two ball bearings. The rigid shaft coupler connects the shaft to the motor. This video shows how it works: