Hide a Folder from Finder
Method One:
Create or rename a folder whose name starts with a period. You'll need to first open Finder navigate to where you wish to create the folder and then press Shift+CMD+Period [ ⇧ ⌘ . ] to show hidden dot folders. These are hidden folders in UNIX and you normally don't need to see them (unless you are a UNIX person or a developer). Now the Finder will let you name a folder starting with a period. Press the same key sequence to hide the dot files.
Method Two:
This one is a bit sneakier but will require using Terminal. Open Terminal and type chflags hidden <path to folder>. To obtain the path to the folder, drag and drop the folder on Terminal and then press Return.
To navigate to the hidden folder. Press Shift+CMD+G [ ⇧ ⌘ G ] and type in the path such as ~/Secrets and press Return and you will end up in the hidden folder.
To unhide the folder open Terminal and type chflags nohidden <path to folder>. You can cheat and type the ~/Secrets. The ~/ symbols mean start from your home folder.
Neither of these methods are very secure to anyone with intimate file management on macOS but it should work for your needs.
Bonus Third Method:
For true security you can use Disk Utility to create a Disk Image and check the box to encrypt it. You'll be required to enter a password. Don't save it to the Keychain or the wife would be able to open it by just double-clicking. Otherwise it will ask for the password. Do not forget the password there is no way to reset it.