As A House Guest, Name Something In The Host’s Bathroom That You Assume You Can Use...

Kleenex

I would certainly assume that I could use a hand towel in there, even if it's fancy. Any towel that's in there, actually, unless there is another bathroom that I'm supposed to be using as a guest.
I would assume I could use the toilet paper also, of course, whether I'm a "house guest", or not. :)

TP

The Toilet

Meds

Their potty paper

I would have said toothbrush but I think only my house guest does that

Flux Capacitor.