Based on what the other classes do when they have 창체 (this is the thing you're talking about, right?), it's kind of a miscellaneous/extra/topics can change type of class. When the 6th grade had it this morning, they had an assembly in the library, then a game tournament, and there was some kind of quiz. They also use it for when the fifth and sixth grades mix together - half of them go outside with the fifth grade teacher for extra P.E., and half stay with the sixth grade teacher for Japanese. It seems pretty random. I've seen them head out to do light maintenance around the school, too - watering plants and such.
You can use this class for phonics, you can probably also pick other topics. You can ask if it's similar to 기초 or 생활, too. My school has me teaching phonics during 기초영어, and the other is a kind of everyday/routine/practical class. I use that one for working up their vocabulary and conversation with specific themes (ex: taste/texture vocab and restaurant roleplays where they asked if this dish was spicy/bland/etc, or game instructions - rewriting the rules for games they knew, then making up their own rules for a board game I drew up).
Edit: Naver says that Changchae is short for "창의적 체험 활동" which google translated to "Creative Activities."
