Here are a couple things I've tried.

Japanese Futon

I spent a year in Japan and used a Japanese futon during that time. The mattress was about 2 inches thick and packed pretty solid with cotton. The futon was usually on a flat, but smooth wooden platform bed. I also sometimes used it on the carpeted floor if I wanted a softer surface. It was really comfortable, but you can't get away with a soft pillow. You need something that gives more support. I had a memory foam pillow at the time. I use a buckwheat hull pillow now and have a very firm memory foam mattress.

Don't get mixed up with American futon mattresses for folding frames, those run closer to 6 inches thick and are as difficult to move as a typical mattress.

Here are a few on Amazon to give you a sense of what to look for. This site also shows a three section foam pad that you can place under a futon mattress if you want something softer.

I saw "throw beds" at Target the other day for around $50. They were basically small, narrow futon mattresses. I can't seem to find them online or I'd include a link. These wouldn't be comfortable directly on a hardwood or tile floor, but if you've got a carpet or rug with some padding underneath, it feels pretty good.

I'd recommend trying something like that out on the cheap if you can before dropping the cash for a Japanese futon and possibly a foam pad.

Camping Sleep Pad

I also have an inflatable sleeping pad that's insulated for backpacking camping. It's really narrow, so it's not as comfortable as a typical bed, but the insulated aspect of it is really nice. It also packs up really small.

I slept on one of those guest bed air mattresses for about a year. I had to layer a lot of blankets under me in the winter or I felt really cold.

I ended up getting one of these: [link]

Which I kind of wanted to before as it feels better on my back, but is excellent for the VR space in my room now.