FYI, you can also use the code as a discount on the Silver or Gold subscriptions, if you want to get either of the subscriptions (you already own 1 expansion after you use the code, so the subscription price takes that into account).
I'd probably go with Adventures, Empires, or Nocturne if I had to pick just one set - those are some of the sets with the most variety of types of cards and card-shaped objects.