We are going to a trip tomorrow and staying the night in a hotel. Don't forget to bring your [...].
I thought about stuff but I'm looking for something less ambiguous (it can be informal).
In a way, it’s a strange thing to remind someone of right after you’ve just told them that they were going to be away for the night. Perhaps it makes sense in some other context, but this sequence seems a bit unlikely:
We’re going on an overnight trip, so don’t forget to bring whatever you need for that night and the next day, since you won’t have the chance to go back home and get it again after we leave.
If you really wanted to, though, I’d probably say not to forget one’s overnight bag. I’ve also heard it called an away bag.
I wouldn’t feel pressed to outline what all I thought they should put in their bag unless it were something one did not normally think to pack, like bug-spray or alternate cell phone or fireworks or rattlesnake antivenin, or even toilet paper if I knew there wouldn’t otherwise be any available where we were overnighting.
I would trust an adult to know what standard things they’d need without coaching. If this were a child without a custodial parent, I would of course be more careful, since children may not be used to packing for themselves.
Given that, you could also say Don’t forget your toothbrush in an informal, somewhat light-hearted way, sort of as a metonym for the whole kit and kaboodle. I have occasionally heard people say that with this meaning.
Speaking of kits, a dopp kit is a genericized trademark for that little bag where you stash a copy of whatever you normally need from the bathroom. Like, well, a toothbrush. It’s the normal word I’ve always heard and used for that zippered little vinyl or leather case, and I was unaware it had any sort of brand affinity.
I suppose you could say toiletries or toiletry kit, but that runs a bit too close to potty training: one often avoids saying “toilet” in any context in America. It’s also getting more detailed than seems necessary in an adult context.
I would say "Don't forget to bring your essentials."
According to Google, the phrase "bring your essentials" is sometimes used when discussing packing for a trip.
'kit'--(British) A collection of equipment or articles including clothes, usually for personal use when traveling. e.g. a travel kit; 'get your kit together, we're moving out'; 'Just one second while I get my kit'. 'I got kitted-out.' (issued personal stuff).'Helen Mirren is famous for taking her kit off.' I'm American but I use this word also.
Just to say "remember your stuff", no matter how fancy a word you find for "stuff", seems only to serve as a warning that you'll be all "I told you so" if they miss anything.
I think you'd do better to spell out what they can expect to find at their accommodations--which, if you're in the position of reminding them about, I'm assuming you have some responsibility for, or at least familiarity with. And "remind" them specifically only of things which they might not realize they even need.
So, "This place provides towels and soap and shampoo." Or, "This place requires you to bring a sleep sack and your own pillow, and recommends we don't go barefoot or leave anything on the floor overnight."
Since this is a single-word-request, I'm going to officially give the new cultural norm answer which somebody has to give in cases where the answer is "There is no single word for that."
So, the answer to your question is:
There is no single word for that.
You'd have to say "your overnight stuff" or "your overnight things."
If your question is about the luggage item usually involved - that could be duffel, overnight bag, grab bag, small bag, overnight case, etc.
If your question is about the small case thing that holds your toothbrush and other toiletries that is called a toiletries bag, or see tchrist's answer. (To be clear your toiletries bag is a little bag that sits in your overnight bag.)
It's a good question and it's interesting there is no single-word (or even phrase) for literally the stuff in question.
The word(s) you're looking for are "toiletries," or "sundries."