Like useful in what way?
Evernote can do tons if you use it right.
Trello is cool for organizing tasks or goals or ideas.
Infinite Canvas or Infinite Painter for art.
I haven't tried it but there's app inventor to make apps:
http://appinventor.mit.edu/explore/
Or at 15 they could actually create their own Android apps and learn Java. Or use Unity3D to create a game or 3D app (
http://unity3d.com/unity/multiplatform/mobile)
If they're musical learn to make music on the phone:
http://www.image-line.com/flstudiomobile/ or maybe a DJ type app?