Scan or photograph your SS card, passport, credit cards, etc. Put the scans and account numbers on an encrypted thumb drive. Keep that with you at all times. This way if your "home" is destroyed, stolen, etc., you'll have numbers, etc. (copy both sides so you have phone numbers for the credit cards).
For the hard copies, keep them in the safe. Don't tape the key to the safe, keep one with you and give the spare to your niece for safekeeping. If you lose yours, you can have her mail the spare (or make a copy of the spare and send that).
As Dutchman suggests, find a way to secure the safe to the floor (preferably to the metal frame).