Here's why:
* Direct Object: The direct object is what is being told. In this case, it's "the best idea for a costume."
* Indirect Object: The indirect object is the person or thing who receives the direct object. In this sentence, Chris is receiving the "best idea for a costume" from Nathaniel.
You can often identify the indirect object by asking "to whom?" or "for whom?" In this case, we could ask "Nathaniel told *to whom* the best idea for a costume?" The answer is Chris.