Maybe a rebus type of thing, which involve questions about the booster pack games. Each answer would correspond to a word or part of a whole sentence. This sentence could say what to write in the comments to win or enter the giveaway. Might want to hold off on spoilers though.

As an extra, the order of the words could be another puzzle.

You could also have just a collection of words and the task is to make a guess for another game with the words as clues.