DEFININTELY CURTAINS!!!!
I have lots of bits and pieces that I put into guest room, mostly that are duplicates gathered over the years. So a decent hairdryer, couple of hot water bottles, spare pillows (in covers) etc all in the wardrobe. And a few decent hangers.
I had thought about cups etc, but thought it might feel like "stay in there out of the way"! So I have a nice jug with a lid (which I tend to fill with water the day of arrivals) and 2 nice glasses instead, many guests seem to want water in bed. A nice chocolate on each pillow would be a treat if a bowl is too much. A small vase of flowers would be nice too (you could always get the test tubes that hang on the wall using suction cups, to put up 2 or 3 nice blooms!!, or other such ones).
I usually put a nice magazine or 2 there. And usually a book of Sudokus/crosswords, a small notepad and a pen. Box of tissues is good, and a bin (with liner in it) if you have a tiny space.
I normally put out a stack of towels on each bed - one body sized, one hand towel and one facecloth. If the bathroom next door is exclusively theirs, then you don't need to do that. (Next part could go in that bathroom instead of room too).
I have a basket of toiletries. Now, a lot of those were from hotels over the years, so things like a matching shower gel, shampoo and soap. Shower cap, shoe shine sponge etc - don't match but obviously hotel so guests are welcome to use them. I also had put together a big lot of travel-sized things that might be useful for DSiblings arriving in the day of DGrandparents funerals (coming from overseas, 3 funerals in 27 months - and as the eldest of 6, living in the city of all 3 DGPs and away from DPs, it was kinda up to me to have the "base" for them to arrive to, and make sure all were respectable too). But I have left it in the guest room as somethings seem to be handy and used up (I know some are overkill).
Cotton buds
Cotton wool pads
Cleanser, toner, moisturizer
New toothbrushes and toothpaste
Razor and shaving foam
Deodorant (male and female)
Fresh comb, hairclips, hairspray and hair gum
Nail file
Nail varnish remover and clear nail polish
Hand cream
Painkillers, antacids, immodium, motilium
Band-aids
Safety pins
Spare pair of plain tights
Spare pair of dark socks
Pocket packs of tissues
Clothes brush
See, too much - but at the time, I was under stress and needed everyone coming through my house to be presentable or able to become so, fast, and not annoy my parents about it either. And not knowing who would be staying, calling en route from airports, not have packed properly, etc.....