With iOS 9.3, you can keep all your most private Notes under lock. First, create a Notes-specific password by going to Settings > Notes > Password. Here, you’ll also get the chance to use Touch ID to unlock your Notes. After the password is set, you will now be able to go to the Notes app and see the “Lock Note” option by pulling up the Share sheet. Once you lock the note, however, don’t forget to actually click on the open lock icon next to the Share icon so that it’s closed. You’ll know for sure your note is locked when you see a message that looks like the image shown here.
The Notes app got a few other improvements, including sorting notes alphabetically or by date created/edited, starting a New Sketch with a two-finger swipe left, and choosing whether photos and videos gets stored only in Notes and not added to Photos. Most notably, you can now easily import from Evernote: Simply long-press on the Evernote Export file to import its contents into Notes.