I love that series: Modern Family :) They mention that they always have the best house during Halloween - meaning that they decorate their house with lots of crazy Halloween paraphernalia. Last year they "took things" to a whole new level. This indicates that they are referring to the Halloween decorations - took things to a whole new level - is used here as a common phrase, to say that they "took" the situation to a WOW factor. Took things is a general term. Took things for granted - is another way to use this phrase. Took things too fast, is also another way of using the phrase "took things"- it's a general way to refer to the circumstance.

”took things to the whole new level“ is a complete phrase, you can't break it down, it means, through everything we've done, we made everything much better than before. take things to the whole new level = we made everything much better than before...