You mentioned soap... just to clarify, I would suggest a bottle of liquid body wash. That can stay in the shower and be used by multiple guests without anyone getting the heebie jeebies.
Wifi password. I leave it on a card in the guest room.
Shower cap.
Sufficient towels - most friends put out two towels, one for me and one for him. I have near-waist length hair, it needs its own towel.
Electric blanket if you live in a cold climate, fan if you live in a hot climate.
Oddly specific but first class airline pyjamas. My husband travels a lot so he requests them in a range of sizes and I put them (new in packet) in the spare room for guests. Our friends and family love them. Often mates will visit but forget to pack comfy clothes for a home movie night or whatever, so they are thrilled with their jammies and love taking them home.