You've been using your smartphone in various ways, but here are some ways you can get it to do even more.
You already use your smartphone in various ways: as a voice communicator (i.e. a phone), as a text communicator (one way as in SMS/text/email messaging, or two ways as in whatsapp), web-browser, GPS (e.g., waze), camera, or flashlight.
Here are some uses for your smartphone you may have not thought about:
Universal Remote Control
Several free apps (e.g., peel, VooMote Zapper, Dijit, or RedEye) allow you to transform your smartphone into a universal remote for your TV (and also your DVRs and set-tops like cable and satellite boxes).
Baby Monitor or Security Camera
There numerouse apps for smartphones that will allow you to tune your instrument, be it a guitar, violin, mandolin, drums, piano, or whatever. Just modify this search to include your instrument.
Your smartphone may even help you make some extra money. there are various apps (e.g., Field Agent, TaskRabbit, Gigwalk) that will connect you to people willing to pay for your time, doing tasks they are unwilling or unable to do themselves.
Instead of using a standalone scanner, you can have use smartphone apps like CamScanner or Genius Scan to scan documents into PDF or JPEG.
You can get motivated yourself without the need for a personal trainer or a pedometer: all you need is your smartphone and apps like runkeeper.
If you're into serious photography, you can save yourself carrying a light meter by using apps like Pocket Light Meter or Luxi.
When you upgrade, resell your old model
When you decide it's time to move to a newer model, don't just leave your older smartphone lying in a cupboard. There's a lively market in second-hand smartphones. Head to sites like USell or Gazelle, and you may recoup a portion of your new model's price-tag.