The current trend is to fill them with colorful, distracting graphics and countless examples of Why We Use Math! and Fun Facts! and suchlike.
I'm gifted in math and it always came easy, but that didn't keep me from inquiring as to why we were learning about imaginary numbers. It seemed pretty useless when I was in high school, and my math teacher did not provide a very satisfying answer. Other children have a lower threshold for motivation to learn math they don't see the point of. These children need inspiration. Ideally, the problems themselves would pertain to useful applications of math. Figuring out how many years it will be before Jill is twice as old as Bill is not interesting or important, but math books are filled with those kinds of questions. Ohms law could be taught in algebra and used to answer similar questions... and the circuits could be built and the answers checked with a voltage meter.
This trend is a sorry example of how algebra and geometry have been diluted in the name of "accessibility." I don't understand why schools can't just teach basic geometry and honors geometry or something like that. But I guess it wouldn't be fair or something.
My school had honors geometry and regular geometry. I think the expectation was the students from honors geometry would never take trigonometry... I know I never did.
