I've put it in books, a mantle clock, odd places in the attic, up on a shelf behind a sink ...
Googling "most common places to hide cash" returns as first result:
1. In an envelope taped to the bottom of a kitchen shelf
2. In a watertight plastic bottle or jar in the tank on the back of your toilet
3. In an envelope at the bottom of your child’s toybox
4. In a plastic baggie in the freezer
5. Inside of an old sock in the bottom of your sock drawer
6. In an empty aspirin bottle in the bathroom (bundled up with a rubber band around it)
7. In the pocket of a particular shirt in your closet
8. In a “random” folder in your filing cabinet
9. In an envelope taped to the bottom of your cat’s litter box
10. In an envelope taped to the back of a wall decoration
11. In between several pages in a random book or two on your bookshelf
12. Buried in a jar in the back yard (my grandfather, incidentally, did this very thing)
13. In an envelope in the glove compartment of your car
14. Underneath a potted plant (or even buried in a small jar in the soil)
15. In an envelope taped to the bottom of a dresser drawer (so you can reach it from the
inside of the dresser below it)
16. Inside of a big coffee cup in the back of a cupboard
17. Inside your Christmas decoration box
18. Inside of an empty bottle of Guinness in the back of the fridge with the cap seemingly in place (smash it to get the cash)
19. In a plastic baggie inside of a flour or coffee container
20. In an envelope inside of a DVD case
If you think you're being clever by using one of these hiding places. Well, maybe not.