So far I've had my insurance and registration with me in my wallet. But I think it should stick with the car in case my wife takes her out.
My RFID toll device (FastTrak) and garage door opener is hidden discreetly under the dash, but accessible enough.
My front plate is under the passenger floor mat (if that ever matters).
Where else do you all hide your stuff? I know none of the above is foolproof, but at least it doesn't scream "take me!"