A fort-building bin with king sheets and clamps
Two king-size sheets and a bunch of clamps in one bin let the kids pull forts tight between chairs and cushions with little help from us.
https://www.afterpickup.com/fort-building-bin/
All the kids love building forts, and they do it often, so we keep a fort-building bin ready. Inside are two large king-size bed sheets and a bunch of clamps, and that combination is the whole kit.
The sheets are big and relatively light, which means the forts can get pretty large without weighing everything down. There is enough fabric to cover a lot of chairs and cushions, and because the sheets stay light, the kids can keep adding on.
The clamps hold those sheets pulled tight between chairs, cushions, and whatever else is handy. We keep a few different sizes in the same bin, and the large ones can hold heavier blankets when the build calls for them.
Blankets, pillows, and cushions from around the house work with the kit too, so the bin does not have to hold every soft thing in the room. The kids pull those in as they go, and the sheets and clamps stay the part that is always waiting.
That is what makes the forts stronger than a pile of blankets alone, and it is what lets the kids build on their own with little to no help from us.