Congratulations! Girls are so much fun. :)
I like
Margo,
Meg, and
Greta, but if I could only choose one I'd go with
Margo.
Margo is intelligent, crisp, and it has a certain style to it. It's a great name, and you don't hear it that often, so she'd probably be the only
Margo in her class.
This one might be a bit of a stretch, but what about
Marnie? Could that work?