How do I ask instructors for more challenging and/or interesting homework?
Why not just ask?  Maybe this seems naive but if you approach the professor and say, honestly, you don't feel you are doing as well as you could or even should be, they would be willing to discuss that; a student taking an interest in their learning is a valuable thing.  You can state your desire for creative questions, and ask if the professor has any recommendations for outside material, or has some of their own.  I can't imagine any professor being anything but delighted at the prospect of a student being self-aware of their performance hurdles and seeking more advanced knowledge in the professor's field.