There is really no “must have” in a rental, as listings run from simple and basic budget-priced to expensive high end luxury. The important thing is to set reasonable expectations for what you provide in your listing ad.
You should also identify your target market. “Must haves” are dependent on your target demographic. Appealing to digital nomads means you should have a proper, comfortable desk and chair, plenty of electrical outlets and good desk lighting. If you are catering to families on vacation, you might provide some kid-friendly amenities like a high chair, a pack n play, a box of toys, etc.
Do not try to appeal to anyone and everyone, it’s best to market towards the guests who will be a good “fit”.
There are a few errors I see new hosts make, which apply to pretty much all types of rentals:
There should be bedside tables on both sides of a bed intended for 2 people, with a reading lamp on each table. Just because a couple shares a bed doesn’t mean they have the same habits. One might like to stay up and read in bed, the other turns his light out and goes straight to sleep, etc.
Make sure your maximum guest count corresponds to more than how many beds you have. If you list for 8, make sure you have a dining table and seating for 8, as well as that 8 can sit comfortably in the living room. Don’t expect 6 people to share one bathroom. Resist the temptation to try to cram more guests in by using sleeper sofas, etc.
Furnish with items that are easy to clean and not too expensive. Guests can be hard on things. Stay away from “sets” of things like dishes and bedding that will require you to buy another entire set if things get broken or stained beyond redemption. For instance, you could go with dishes that can be purchased by the piece to match what you already have, like at Ikea. Keep spares on hand, packed away, so you can quickly replace if you find the guests have broken 2 plates out of a set of 8.
You could go with all white bedding and towels, so if your bottom sheets wear out before the top sheets, you can just buy new fitted sheets. Or a color scheme, like various shades of grey, or blue, or whatever, so you can mix and match and it still looks purposeful.
Stay away from purely decorative items, aside from wall art. Guests aren’t interested in knicknacks or clutter- they want a place to put their own stuff without having to move a useless vase of dried flowers or some little statue. And you don’t want to increase needed cleaning time with shelves full of things that have no practical purpose that are just dust collectors.