What are good ways and places for storing backup of an encrypted wallet?
When somebody open encrypted wallet what all can he do when he does not have a key? asuming that the key to the wallet is e.g. 73 trilion years far to cracking
You can back it up pretty much anywhere. USB drive, backup hard drive, Dropbox.
If your wallet is encrypted, all they can see is what addresses you own and how many bitcoins they have. They cannot send any bitcoins or get the private keys.