Think about who will most likely come to stay. Is it friends for a weekend, family who live abroad for longer visits, people with dc, a long term au pair???? How often will the room get used by guests?
Our guests get
Double bed (ottoman storage bed, it's brilliant as you can store so much stuff underneath it all hidden away!),
memory foam orthopaedic mattress (from groupon),
100% white cotton bedding (tkmaxx).
Large bath sheet sized towels and a smaller hand towel.
There is a bedside table with lamp on it, a couple of books, and plug converters for my family from Europe.
The en suite is stocked with usual bathroom stuff (showergel, shampoo, conditioner, comb, cotton buds, handwash, etc). We also have spare toothbrushes in there for people who stay unexpectedly.
As our friends started having dc we now also have a travel cot and the mattress from our dcs toddler bed stored so when they come to visit there is enough for their dc to sleep on plus a nighlight. All this is hidden in the storage bit of the bed when not needed.
We have guests visit once or twice a month so our room is used pretty frequently - however I felt it was wasted just as a spare room so we've put a projector in there and now love family time snuggling in the bed watching Disney movies. I also allow my oldest dc to build lego creations up there that the youngest can't get to destroy ... it means the room gets more use even when we don't have guests.