Better said, you could substitute smartphone for pocket personal computer with sensors and camera and this bundle is the amazing part. Any of these following uses come without emitting or receiving microwave radiation, in the so-called "airplane mode".
Measuring distances / spirit level / other diy
There are applications that approximate the size of objects like the Eiffel Tower or smaller ones like trees. Others for technical measurements in rooms for diy activities. The aforementioned spirit level is like those tools that you could buy around there, it uses the bundled accelerometers, the app draws normally an alike bubble.
Sound recording / security activated by sound / metronome / tuner
Thanks to the internal speaker and microphone all these activities are ready and usually at high accuracy if the device has quality parts. There's also room for improvement using the micro-USB and 3rd party hardware but this feature is still not well planed but at least some options exist.
The surprise is how the device does a better job than some dedicated hardware.
Money, credit card substitution
Companies based in ease and trust could establish (they are but the adoption could be faster) a network of payment processors in a RFID style in any place with previous authorization codes like the PIN typed by the user before authorizing a payment.
For who pays, Internet connection is not necessary since the receiver can do the appropriate confirmations. It can speed up the process considerably with all the benefits for both sides.
GPS positioning, marking
Applications like osmand for Android provide very detailed offline maps that once are downloaded, the user doesn't need Internet connection. It permits to know your location even in the middle of the desert without more than receiving GPS data.
An extra tip for achieving a GPS fix in seconds:
• Configure your GPS with a server in your zone, normally the same used for domains (.au, .fr, .br and so on). It's a good practice to have two gps.conf versions one with mainland based servers and other local just when the local doesn't work as expected
• Use an application that permits A-GPS download like GPS-Test
• Disconnect plane mode, activate Internet for a mere moment Internet to download A-GPS. After, disconnect Internet to avoid any emitted radiation near your cells.
Lantern / long distance signaling / warning lights
Very useful sometimes, via the screen or the LED.