Hello! Jem and Scout are incredulous, but happy as only children can be, to find secret treasures in the knot-hole of the tree. Jem asks Scout if she ever notices that they only find the treasures during the school year. When they find two Indian-head pennies in a purple velvet box in the knot-hole, they are excited. Scout asks Jem whether they should keep them. Jem reminds Scout that they don't know who the owner is: therefore, they couldn't return them even if they wanted to. In the end, Jem puts the pennies in his trunk; he tells Scout that when school starts, they could ask around if the pennies belonged to any of the other school-children. However, the novel is silent as to whether Jem and Scout actually do this.
Later on, after the summer, the two children find more treasures: a ball of gray twine, two soap dolls made to resemble Jem and Scout, a package of chewing gum, a spelling medal, and a broken pocket-watch with an aluminum knife on a chain. Jem puts the soap dolls in his trunk, and tries to repair the watch. However, he is never able to get it to run. The two children enjoy eating the gum, and towards the end of the novel, Scout tells us that besides all the treasures Boo gave them, he also saved their lives. She is regretful that she and Jem were never able to give back to Boo equal value for what he gave them.
Hope this answers your question.