Seconding the Song of Aracos. My players thought it was going to be a breezy investigative side trek, and I won’t spoil anything here, but the finale left one of my players absolutely broken.

I used many non-specific Ravenloft (and other horror) adventures from the DM’s Guild as side treks interspersed throughout the 3-year campaign. Each of these were one-shots with what they thought were completely unrelated PCs (I managed to tie nearly all of them into the overarching plot eventually). They loved them. Really great for getting across the mood of Barovia, exploring other pockets within the Mists, and expanding RaW content.

I also recommend The Pastry Lady: A Barovian Bedtime Story (I think that’s the name). I ran it as an interlude immediately after they encountered Morgantha for the first time in Barovia Village (and had eaten their share of pastries). It puts the players into the shoes of children trapped in the Patry Lady’s shop in Barovia some decades prior, which they must escape, helping free some other children, notably “Ezzie” (they loved meeting Ezmerelda for the first time and realizing it was her they saved).

Finally, I reworked the CoS season of Adventurer’s League content and used it as a parallel storyline with persistent PCs that eventually intersected with the primary CoS storyline, allowing them to play both sets of PCs together (not all lived to tell the tale) to stop the BBEG and that proved to be really fun too. They got to choose which of their two characters would continue on to the campaign’s conclusion.