If the keys are small could hang them from seat post and hide them in the seat tube.
Or possibly hide them in the steerer (top of fork).
Not very secure but tack weld the key ring shut and lock the ring to bike.
Let the air out of a tire and hide the keys in the tire but now you have to pump up the tire and someone might steal your bike pump.
A water bottle type safe where you can only get to the screws with the safe open.