I don't know about cash, but the spare key to get into the house - we used to have a pet rabbit, we hung a key hook just inside the door of the rabbit hutch. Whenever the door was opened the rabbit would rush to the door and scratch madly at the hand tat was there, unless you knew to pull your hand back for a minute and then pat the rabbit on the head and scratch him behind his ears. So if you didn't know the rabbit well, even if you knew the key was there you wouldn't go in after it.
When the rabbit died (old age) our next hiding place was in the chook house, underneath the water bowl. This was muddy, there were lots of wood lice and slugs. The contents of the water bowl were putrid especially after a hot day. We finally had to stop leaving the spare key there because it corroded away.
And I'm not telling you where the spare key is kept now... oh, alright. We have a small fake rock. It's hollow, made of polyester resin and looks exactly like all the other rocks in the rockery. On top of this, we have buried this particular rock under another, so even if someone recognises this particular hollow 'rock' they won't see it. So to get the spare key, you have to know where to dig.
Another really good hiding place I saw the other week when I was in the city buying shoes, was a fake can of baked beans (or similar). It unscrews so you can hide your keys, money or whatever inside it. Anyone breaking in to your car is unlikely to steal a can of beans. I nearly bought one for difficult child 1, but at A$25 I thought it was robbery!
Marg