I have another in mind but haven't tried it. If they leave their phone unattended, take a picture of their phone sitting there. Then later on text them the photo and say "You left your phone (wherever)" You will get them to view the photo of their phone on their phone and if you're lucky they will go looking for it when they already have it.
If they use android, you can open the camera without unlocking the phone. Just double tap the power button, and it pops up. It's intended for rapid camera use, but great for pranking purposes. Apple has the equivalent, though I can't remember the button combo, off the top of my head.
Wow. Been using Android since like version 2 but never knew you could double press the power button to turn the camera on. If screen is off have to press once to turn screen on first.
IIRC on iOS there is a camera icon on the lock screen.