What did Sophie bring back from her camping trip with Joanna?

In "Sophie's World" by Jostein Gaarder, Sophie brings back a wooden box from her camping trip with Joanna. This wooden box contains another envelope from Alberto Knox, her mysterious philosophical mentor, which is an essential part of the ongoing philosophical narrative and education that Sophie is receiving throughout the book. The envelope contains philosophical content and prompts that lead Sophie to continue her exploration of the history of philosophy.