A child's true intelligence development does not rely on packing their schedule full, but on two things: moderate difficulty and moderate boredom.

Difficulty forces children to think actively and solve problems, causing the brain to continuously form new neural connections;
while time spent in boredom and daydreaming, seemingly doing nothing, actually allows the brain to freely associate and build imagination, which is precisely an important process for the growth of creativity and abstract thinking.
So, don't push children too hard; what truly promotes brain development is not just inputting knowledge, but giving them space to think and explore on their own.