Shaving or grooming your balls is actually something I find very sweet (if you are able to and it doesn't throw the SO off). There's nothing like a slick sack to twirl around on my tongue.
If you can't get away with that for whatever reason or already have a shaved sack then flowers or a small gift card to victoria secret in your gift envelope is a great way to say thanks.
Honestly though, the fact that you are complimenting her with a multihour first meeting is flattering enough in itself. Don't over think it. Going with the flow amd being yourself is always best