In the order guests actually notice
Guest rooms tend to get decorated from the outside in, which is backwards. A guest notices the mattress, then whether there is anywhere to put a phone, then whether there is a light they can reach. The decorative layer is noticed last and costs the most.
1. The mattress
Spare-room mattresses are usually the oldest in the house. A topper is the cheapest meaningful fix and it changes the one thing a guest cannot ignore. Check the depth against the fitted sheet pockets, because a topper can put a mattress beyond what the sheets will reach.
2. A surface within reach of the bed
Somewhere for a glass of water, a phone and glasses. A small table, a stool, or a wall shelf all work. Without one, all of it goes on the floor.
3. A light they can turn off from the bed
If the only switch is by the door, your guest has to choose between the dark and getting up. A bedside lamp resolves it. Check the cord length against the distance to the nearest outlet.
4. Two pillows and a spare blanket
People sleep differently and nobody asks for another pillow. Two per guest, one firm and one soft if you have the option, and a folded blanket at the foot so extra warmth does not require a request.
5. Towels on the bed, not in the cupboard
A stacked set on the bed tells a guest which towels are theirs. In a cupboard they will not find them and will not ask.
What to skip
A full decorative pillow arrangement. Your guest will put it on the floor, and now they have to remember where it went.