A few years ago, I had one of these plastic credit card-shaped keys:
AAA in California provided it when I joined. Never had to use it, but I've read that they do work. A problem, though, if you've got the type ignition key with embedded chip. And if you lose your wallet/purse, you're still up-the-creek without a key.
This one allows you to store a spare key in a compartment integrated in a combination-operated locking gas cap. Great if you don't have a locking door over your gas cap
Then there's the old standby, the magnetic spare key holder. Seems like there are enough nooks and crannies on car's undercarriage that all but the most determined thief would have a tough time locating a well-placed container.