InstantDrummer is laid out very similar to FourTrack, so again, navigating around the app is extremely simple. There are a variety of InstantDrummer apps that you can download depending on the style of drum groove you're looking for. The cost is only $2.99 (USD) each and the sounds are phenomenal for the price. With InstantDrummer you can create your own drum arrangements for a song, and then add EQ and compression during mixdown to get the perfect tone/punch that you're looking for.
With the Audio Copy/Audio Paste feature, you can then copy and paste the drum grooves from InstantDrummer into FourTrack.
If these two apps aren't exciting enough, just wait you spend some time with GuitarTone. This is where it really starts to get exciting. GuitarTone, along with GuitarJack ($129 USD), in my opinion, is the quintessential guitar app/interface for mobile devices. With GuitarTone, you get a variety of modeled vintage and custom amps, as well as pedals, cabinets, and microphones. Again, the interface is very intuitive and user friendly, allowing you to quickly create the guitar tone you're looking for.
It can be accessed through FourTrack, ideal for recording a great guitar sound on your FourTrack song. GuitarJack is a high quality 24-bit interface, allowing for a guitar input and a stereo mic/line input. The mic/line input includes pad, normal and boost modes, with 60dB of continuous level control. This is perfect for recording simple vocal ideas with a dynamic microphone.
GuitarTone is $2.99 (USD), however if you're looking for more sonic freedom, there are add-on amp packs and pedal packs for only $9.99 (USD) each.
For less than $150 (USD), you can have a complete mobile studio in the palm of your hand. Gone are the days where you needed free space on your desk for your recorder, drum machine, and guitar processor, not to mention the added mess of cables. With this group of apps you will definitely get some great sounding recordings and demos of your songs. Even as a simple practice tool while traveling or on vacation, the GuitarJack interface and GuitarTone app are an incredible value.
One last thought. Don't overlook the Audio Copy/Audio Paste features of some of your other music apps. FourTrack will paste in the audio from any other app that enables Audio Copy.