Offline. Put the wallet .dat files on a usb stick and keep them safe.
Encrypt the wallet.dat, and then put it on a usb stick.
It helps if you also rename it. Possibly if the .dat doesnt destroy the filesystem, then even put a .txt extension, but not recommended.
An even safer way is to encode the content of the wallet.dat, into a BMP or JPEG file, and keep it in image format, nobody will look after that.
Wallet.dat files require that the future program be compatible reading the file and the file not be corrupted. For short term this might be viable but anything spanning years I would recommend going back to rudimentary printing the keys on paper and dispersing them. I prefer X of Y paper copies of my keys like the kind made by Armoury.
Coinbase is great for people who already realize that it's futile to avoid being tracked in large cities where cameras are watching you 24/7 and phones are constantly dialing home. Do they really care I used a restroom in a Applebees on Friday?
