I have a spare housekey hidden near my residence, but not actually on the property. This idea I got from geocaching.
What I did was take a standard metal coathanger, snip & straighten it, hammer one end flat, and form that end into a hook. So, it resembles an upside down J. At the straight end, I taped a set of spare apt keys to it. I then went to a storm drain on my street, up a little ways from my apt, and dropped it in, with the J part of the hook over the grate. It blends in perfectly, and no one EVER looks in the storm drains. I check on it when I go for a jog, and pull it up once a month or so to ensure its still attached. Even if someone finds it, they would have to figure out who it belongs to then which door inside the complex. The downside is that there is always a potential of losing the keys-but, this is a relatively small price to pay I suppose.
I cannot lock myself out of my apt, as the door has to be locked via key. My jeep is a softtop, so locking myself out is a non-issue. However, the backup keys are in the event I lose my keyring-I have dupes of my apt, jeep, & gun safe on there. The gun safe may be an issue to have another dupe made, as I have to contact the manufacturer directly.