A theme I feel is never brought forward enough in the series is choices and the effect they have. Snape choose dark arts over his friendship with Lily, and eventually it cost him her as a friend and it killed her. Lily chose to die for her son, and it gave the wizard world the chosen one. Voldemort chose which wizard he thought best fit the prophecy, and destroyed himself in the process. Choices, and the consequences that follow, is a great theme in this series.
i heard angus liked mysteries so i gave him a big book of riddles from when i was a kid. two hours later he gave it back to me with every riddle solved and annotated. he was so happy that he could help me and i don't have the heart to tell him that's not how riddle books work