Sans is absolutely getting teleported into the lake. He can swim, but in his normal body he didn't have to breathe and didn't feel cold as sharply, so this will be a learning experience.

Sans will mostly be holing up at home once he realizes things are going down, with only the bare minimum of excursions to get food/maybe go one house over to check on Papyrus/go to work enough not to get fired. He might drop the work thing if it's dangerous enough out there, honestly. Sans is definitely going to find himself teleported to the Christmas village a lot to make sure he gets plenty of outdoor time in spite of himself. He may end up being chased around by teleport portals as he tries to avoid getting sent to the village again. Expect him to climb out at least one window. I'm not sure if I'll be doing anything with the reindeer yet; it depends on the specifics.

I'm definitely interested in doing something with the snowmen; psychological horror is always fun. If having them around is dangerous, he might start knocking them down at least on Loomis Drive, but not if knocking them down is also dangerous.