Spare room

We bought a bunk bed set with a double on the bottom and single above - seems to cope with any configuration. If a twin room was necessary they could have our bedroom, while we decamped to the back bedroom.

I have one room with link beds that can be either two good sized singles or a king sized double.

If you have space, 2 singles can be more versatile than a double.

We bought a single bed/day bed with another bed underneath that pulls out with legs so can be used as a single, 2 singles or together to make a large double. It's very versatile and comfy.

Really depends on how much room you have. I hate sleeping in single beds, (will pay extra at B&B's for a double bed), so I have a double bed in my spare room, but then, upstairs in the loft extension have two single beds and room for a blow up mattress.
My spare bedroom is very comfortable, with bedside lights both sides, clock/radio, fan (for the summer), TV, plenty of points to plug things into and a cupboard which is used for towels mixed sizes and spare bed linen, also have a note telling visitors how to set their 'phones etc on my wifi.