Hmmm, I've been going back and forth about this topic in my mind trying to figure out if I think of myself as creative or not.
Cimarron, I can relate to what you said about instruments. I played the piano for many years, and although I was a technically good player I did not create my own music, nor could I spontaneously put something together.
However, I did always really enjoy art class and playing with color, and drawing and stuff...and I do like abstract art. Not to say that I was ever fantastic at art, but it is something I still enjoy. I did some sculpting (with clay) too...which was lots of fun.
In the sense that ISTJ's often have dry/sarcastic wit about them, I think that takes some sort of mental creativity. A well-placed punchline or comeback takes quickness and the ability to play with words. To me, that is creativity.