Disk Utility does the trick!
This isn’t anything new, but still works as an easy way to keep files away from prying eyes.
I was looking for a way to password protect files that would be transported on a USB stick and thought maybe it could be done at the folder level or even for the whole USB stick. Another option is to create a password-protected disk image and keep your secret files in there. The other options may work fine, but after a quick test with a disk image and I didn’t even follow up on them.
A Macworld article from way back in 2004 explains how. The section on “Encrypt Secret Files” goes through the steps. So I actually got a bonus: encryption and password protection.
The overall process is simple: use Disk Utility to create a password-protected, file-encrypting disk image .dmg file (see the article section mentioned above). Then to mount it, you just double-click and enter your freshly created password. Now you can simply drag the files you want to protect into the mounted disk. When you’re done, you eject the mounted disk. The disk image file with .dmg extension is still there for you to place where you want (like on a USB stick if you’re on the road) and mount again when you want to access your files. (The encryption and decryption happens without you having to do anything other than click a setting when you create the disk image.)