Scout discovers the first 'gift': gum.  She promptly devours this and only later confides in Jem that she found it in the tree.  Other gifts follow in chapter four - two Indian head pennies.  Then over the course of the next few chapters, they discover more gifts - a ball of twine, two hand carved figures that represent Jem and Scout, a medal, and finally a pocket-watch and a pocket knife.  These, of course, are left as tokens of appreciate from Boo, who obviously has been watching the kids and their plays and games.  In this way, he appears to find them as fascinating as they find him.  The gifts however, come to an end when Mr. Radley has the hole filled with cement, cutting off Boo's means of communication.