The first DLC, Hidden Dreams, added a couple of "dream bosses" to the game, which are more challenging versions of existing bosses. (The second dream boss added, The Grey Prince, is actually an entirely original boss, unlike the five or six others already in the game). They're infinitely repeatable, unlike every other boss in the game, and get harder with each victory, eventually scaling up to dealing absolutely comical amounts of damage per hit after seven or eight wins.

The second DLC, the Grimm Troupe, added a short sidequest and a few more unlockable charms (think Rings from dark souls), as well as one new boss and a corresponding dream version, and an extended platforming section that's hidden in the already existing platforming challenge area of the game. The Grimm Troupe content is considered to be the hardest content in the game by a pretty wide margin.

Neither are particularly big (maybe 4 hours of content each at most), but they integrate rather well into the game, and they're altogether quite enjoyable.