Simplest way must be a password protection. The most secure way may be the hardware wallet.
Actually, hiding the private key in the garden is not a bad idea, as long as it's stored correctly. People are crazy about their bitcoins
Simplest way to keep bitcoins SAFE is to generate and print a paper wallet offline, or keep the bitcoins on an offline computer.
Simplest way to keep SAFE is actually not printing a paper wallet -- it's coming up with your own long pass code. Something like "When I was nine, I fell off my bike on the way home from school" -- things like this are almost guaranteed not to get cracked. And then you need no paper wallet because you just input your code when you need to send funds.
On paper wallet I highly suggest if someone uses them to get rid of some of the nicer designs. Just keep what you need (just cut with scissors) don't have bitcoin written all over it as I still don't think standard thief will know bitcoin QR code. Could even put something to mislead them like "Family Pictures" with the QR, etc needed. Chances are they could care less on family pictures if that's what they think it is.
Also you could use a usb stick with paper wallet stored on it. Put encryption with passphrase like Ranlo mentioned. He is very right a passphrase is very very hard to a bad guy to brute force compared to password just a LOT more possibilities.
With my method, you have no trace. You don't need paper, a USB stick, or even a PC. You simply use the passphrase to sign transactions when needed. Nobody can steal what they can't see, even if you're robbed.