From Quora, here are some cool hidden features on your iPhone / iPad:

1. Offline google maps by searching "ok maps"
2. Swipe in the calculator display to delete a digit
3. Turn on field test mode to show signal information by dialing *3001#12345#*
4. Double tap a word to select, quadruple tap to select a paragraph, hold one finger at the beginning of text then place another finger at the end to select, double tap but hold and drag second tap to select more text
5. Use Guided Access (in settings) to disable buttons or parts of the screen
6. Make the camera flash like the Blackberry LED: Setting -> General -> Accessibility -> LED Flash for Alerts -> ON
7. Kill many apps at once by swiping up on multiple apps
8. After selecting a word, tap Define to get the definition
9. Add textexpander style shortcuts, i.e. to insert a specific bit of text, like your email address, type just a few characters. I have mine set to "@@" - Settings > General > Keyboard > Shortcuts section

More here: