I have a bright 9yo (homeschooled) who does best with a mix of formal and self-directed learning. Subjects like Latin, formal logic, history of the English language, seem to work well with scaffolding--at least for my very analytical kiddo. I think it can be very satisfying to wrestle with progressively harder problems, and there are some fantastic materials out there with genuinely interesting problems.
That's not to discount the value of fallow time, in which I also believe, but I do think that there are great things to be gotten from a lesson plan at that age, if it is more practical to use one in that situation.
For schooled children, I have seen the recommendation for advanced work to go deeper, rather than ahead, when possible (so instead of doing algebra, read about history in math or number theory) OR to try doing advanced work in subjects that are not part of the normal curriculum (another language, diagramming sentences, a branch of science that isn't going to come up soon) so that the student will not then be bored in two years when that material does come up.
A few suggestions:
anything by Ellen McHenry--mapping the world with art would work well with no supervision needed
Jacobs: Math the Human Endeavor (and Art of Problem Solving books when that is outgrown)