I periodically get asked a form of this question over the year. As is my usual default, I mostly inform people about either security or privacy settings, assuming I know they have backups enabled and that sort of initial thing. Being mostly an Apple user, with my friends and family generally in the same category, I recommend the following (ca. iOS 11):
set a passcode — preferably a password with longer length, but some is better than none
set up Two Factor Authentication (2FA), and maybe set a second trusted phone # for your account to your spouse/SO — some think it's a PITA (Pain In The *Backside), but it's worthwhile
2FA/MFA (multi-factor auth) is a good idea to enable most places, especially for banks, social media accounts, or anything with access to your other accounts
enable "emergency sos" — be sure to set whether you want "auto call" to kick in or not; not only will this give you an option of clicking your lock button 5 times to enable for medical and/or auto-call (if configured), but it also requires your full passcode to unlock
enable Do Not Disturb While Driving — I don't care how careful you think you are, you're better off not touching your phone while driving; Siri can more than do enough when connected to bluetooth, and aftermarket setups are plentiful and inexpensive, like this one
enable iCloud backup — most people should be able to work within the free iCloud account storage space of 5GB
enable find my iPhone — otherwise you won't know you need it until it's too late; should you find yourself in a pickle, you can use the icloud.com website
I periodically get asked a form of this question over the year. As is my usual default, I mostly inform people about either security or privacy settings, assuming I know they have backups enabled and that sort of initial thing. Being mostly an Apple user, with my friends and family generally in the same category, I recommend the following (ca. iOS 11):