Frisk will mostly be spending time trying to adjust to life in the base/ocean, which is difficult given the nightmares.

Re: the nightmares, Frisk will still be sleeping, since they're a kid and need their sleep. They won't be very happy about it, though, as helpless, isolation, and loss of self can all play into fears they've had. They'll also definitely get chased by dogs, though that's not too scary.

Re: Halloween, Frisk will be dressing up as a skeleton! They'll need someone to help make this costume. Also, there had better be a trick-or-treating party log. We can't have Halloween without trick-or-treating! They'll also want to help with decorations!

In general, Frisk is up to help wherever! They like helping.