She'll know what it is and be mindful when opening it in public

Either get a "thank you" card or just an envelope and give it to her OR wait until you are in private and place it somewhere that is highly visible. She might want to open it up and count it.

Or if it's not too bulky, a greeting card. I had one gentlemen tuck an envelope into a book -- very clever! Unless otherwise agreed upon, it's best to default to conducting the financial stuff at the beginning of the date to avoid any awkward "So... did you bring me something?" exchanges.

Or when I met a gentleman for coffee he suggested I read this "great article in the newspaper on page blah blah," by the time we reached the hotel it was green light.