Based on what I can observe from my INFP friend, @kaleidoscope , I find the INFP imagination to be a thing of wonder. It's not all puppies and rainbows and yet there is something distinctly beautiful in the way INFP connects his/her thoughts and ideas based on the surrounding environment.
I had been listening to some music earlier in the day and stumbled upon something that reminded me of the INFP imagination (a theme song, soundtrack, or what have you) and when I did meet up with young Miss Kaleidoscope, I shared my song of choice for the INFP imagination with her:
This is, of course, only my opinion. Feel free to share yours (INTJ consent).
Oh, and if I had a say in the sound of the INTJ imagination/thought process, I would have to pick this: