For double chocolate, I used vanilla flavoring and placed white chocolate chips on top.
For peppermint, I used peppermint flavoring and placed junior mints on top. Crushed peppermints would also be a great topping.
For walnut, I used vanilla flavoring, mixed in chopped walnuts, and placed chopped walnuts on top.
For the coconut almond, I used coconut flavoring, and placed a whole walnut on top of each square.