Certainly you realize by now that when a woman says, "I don't mind", she's lying! Last time I took Marge to Burger King, I couldn't see out of one eye for a week.
What she means is, "surprise me".
Take her to a mid-range, but somewhat classy place. A hole-in-the-wall restaurant might be OK if there is something special about it, like it has a really good cheese-steak, or a famous person used to work there, or it has a funky decor that makes for good conversation.
But don't go for too expensive, because that implies that you are too serious about the relationship too soon, and creates an awkward vibe. Save the expensive place for her birthday.
French is usually way too expensive for a casual date (I know of a few exceptions, however). Just take her to a place that YOU really like, so that: (i) she gets to know you better by seeing what places that you like, and why (women like to "discover" things about you); and, (ii) it insures that you, at least, will have a good time, no matter what happens.