At 9 years old, my son is just on the outward bound of where we can absolutely make him lose his shit for a hundred bucks worth of Pokemon sheets, wall stickers, and a utility strip:
We did this while he was in school, and when he saw what we had done, it absolutely melted his mind. Like I did when my own parents gave me Star Wars sheets in '81. Most rewarding return on a hundred bucks I've had in years.