Milo, like many smart children, is bored in school. He does not see the point in learning things: he thinks that a lot of what he is being taught is obvious—such as "subtracting turnips from turnips"—and the rest is too irrelevant, like where Ethiopia is. The narrator tells us that he "didn't know what to do with himself."
A crucial factor here is "time," which Milo sees as being wasted. When the mysterious package arrives, its message names this condition as "excessive time." On its label, addressed to Milo, it says he "has plenty of time." Along with the rules and regulations, the instructions tell him that, if he is dissatisfied, the time wasted "will be refunded."
When Milo returns from his adventures, he has been intrigued and now expects to continue to find life interesting: he is no longer bored. Looking around his room, he thinks about "all the puzzle and excitement of everything he didn't know."