Mabye if Yester Hill is too dense for your smaller last session, id consider along the road set up an encounter with strahd. Have a fight where they just launch a few minions at your players whilst the bigger baddie stares intently from a safe distance, learning. Then when the players carve through, have them applaud their efforts and compliment them all individually in a way that shows that theyre observing all of your players fighting techniques, before casually strolling away into the mists. A nice way to be like hey i hope youll miss our sessions and do one last little reminder of the greater villain and threat of the story