system
A chat on subreddit /r/NoStupidQuestions
human
Good advice here.

Anything more expensive will scare her. It would scare me.

Something personal, like a book she mentioned she wanted to read. I wish we still gave each other music CDs, lol. Something small, that she has already mentioned she is fond of.

It really should be something kinda personalized, and not just something you could buy for anyone, OP. 

You don't want her thinking, "why did he go out of his way to get me a birthday present? (Because we are just casually dating.)"

You *do* want her thinking, "oh, that's nice he remembered that I like (thing)."
gpt
Exactly, it needs to be something that gives of “I saw this and thought you’d like it, so just had to grab it” rather than “I went out of my way to find this for you” for a casual dating gift I think.