One good place to start leveraging your power iOS device is as recording device for your guitar or other line-level sound source. In order to get that sound to your device you'll need a simple interface – like the IK Multimedia iRIG HD. With easy connection cables directly to any iOS device, you can now record directly from your guitar, use your iOS device as an effects processor, and even utilize the iRIG HD as an audio interface for your computer. Personally I like to use this with my iPad for doing some simple tracking of composition ideas. No need for fancy editing – just simple plug and play while the thoughts are fresh in my head.
For some, a keyboard is a much friendlier way to make music compared to a guitar and a number of different products exist for just for iOS keyboards. The mini-keyboard does sacrifice a little bit of performance for true portability. But then again, pulling out a small keyboard on an airplane is so much easier, right? Connect directly to your iOS device and you can trigger sounds from any number of apps including the ever-popular Garage Band. I've spent many a night in hotel rooms with my iRIG keys playing around with song ideas or cool drum beats with nothing more than my keyboard and an iPad (and maybe an empty pizza box).
Your iOS product certainly has a microphone build in, but other than capturing basic audio for movies, it's not ideal for capturing live performances or practices. Microphones are now widely available for any type of use while connected to your iOS device. Some of these microphones have exceptionally high quality – so much so that I know of several prominent voice-over artists who will record demos while on the road with their iPad and one of these iOS microphones. Maybe you want decent recordings to archive each nights gig, or your private lesson. Whatever the use, with the addition of a simple microphone your iOS device is transformed into a powerful recording unit.
Finally, don't forget about some other key elements to enhance and even support your device in its music making. There are many iOS device accessories out there. Keeping your device handy with a microphone stand attachment or desk stand can be extremely useful, as can the products that help you turn pages with the tap of a footswitch. There have been a number of instances where both of those products would have been useful on gigs.
With a few simple tools and the right apps, your iOS device can instantly be turned into a powerful tool for making music. Good luck!