The Problem with Barovian Witches

As most folks here I'm running the modified version of the dinner at castle Ravenloft. Our last session ended with the conclusion of the dinner and the PCs have been extended an invitation to stay the night which they accepted. Now, the problem is that Strahd promised that they won't be harmed during their stay and he intends to keep that promise. This is where the Barovian Witches come in. As written, they should try and subdue the characters during the night which would be a direct defiance of Strahd's will. At the same time I would still like the night to be eventful somehow. Any ideas what to do? I suspect the players might go sneaking around but they will eventually return to their quarters unless something goes terribly wrong. Any suggestions are welcome.