The safest thing to do is take out the SIM card and put it in something like an SD card case, the kind you get when you buy SD cards. In the same case, keep a safety pin in it to use when popping the SIM tray out to remove and re-insert the SIM. Keep this case in your wallet or purse and always know where it is, when you decide to put the SIM back into your phone.
There will absolutely be no mistakes as there is no physical connection to your cell carrier. This is really helpful if you let your kids play with your phone and they figure out how to switch airplane mode off.
Wifi will still work. So in wifi zones, you can make calls using the app Talkatone (free) and text using TextPlus (free). Note both apps do not require the receiver to have the app or even a smartphone which is nice.
That worked no problem in our hotel and at the airports.
Because the iPhone has a GPS chip, full GPS apps also work without a SIM. The maps are stored locally but the full-featured apps are not free, eg; TomTom.
Wifi on
DCL is expensive and really slow, I am not even sure it will support VoIP like Talkatone.
I'm from Canada, so roaming charges for voice and data are very expensive. My other options beyond the above are to pay for a travel package from my CDN carrier, or, buy a U.S. SIM in my unlocked phone and add PAYG minutes/data.