Impro Ball for Android
There are 3 different levels of difficulty: easy, medium and hard.
In the easy level, the game randomly picks a key of the chord so you have to play
that key. You have only one path, and that's not really the goal of improvisation,
but you can get familiar with this app.
The medium level still has an unique path, but it also shows you the other keys
of the chord. You can play them as well and try to make some nice melodies !
The hard level doesn't show you the unique random path. You are completely free
to chose which key you want to play.
You can see the labels of the keys and chords by selecting Pro Mode in
the option selection or even change the appearance of your ball !