Furthering one’s knowledge (which BTW does not require an AP exam) because they have exhausted their HS curriculum is one thing. And a totally different thing.
However, this situation does not apply to the OP’s daughter, who has yet to set foot into a HS classroom. The fact that she does not yet know how much she does not yet know, combined with the fact the she is looking for the easiest AP’s to self study, is why I am not jumping up and down to provide suggestions, which BTW are easily found via Google as they’ve been asked a million times already. When she’s completed the most demanding science courses in her HS, I’ll be happy to suggest resources for quantum physics. 