I use my iPhone's Notes app to save everything from book and music recommendations to a password list. Obviously I have my phone password protected but if you need an extra layer of protection with your notes consider adding an additional lock to specific notes.
This is great particularly, if you are like me, and hand your phone to your children...who don't need to see that information.
It is super easy to do. See the step-by-step flyer I made above.
A few items to note:
1. Whatever you set as your password for one note applies to all notes you attach a lock to.
2. Once you unlock one note, you unlock all of them.
3. You have to close out the app or put the phone to sleep before the lock resets.