Turn your leg into a piano. You never know when you need to jam out.
When you sit with your leg crossed over your other knee (in a manly way, not in the girly way, you know what I'm talking about), your leg lies nearly flat.
As for the pants: Capacitive cloth with a piano pattern printed on it could render your leg a useful surface for a cloth-piano.
Could put a speaker somewhere idk. At the belt? in the shoes?