For me, language exposure (careful selection of word over "learning) trumps all as a creativity-boosting activity. Based on personal experience, how creative you become is a function of how much novelty you are receiving. Language exposure (through whatever means best suited and liked by your child, watching shows or playing games in a foreign language etc.) is that activity which gives you the greatest amount of novelty as you are constantly being exposed to the new and unfamiliar, especially when it comes to vocabulary.