1. Build a cave
A cave can be many things. They can be quite simple and they can be huge, containing a ton of things. A general rule of a cave is that it takes up a lot of space even when small. It is physical, it gets messy and needs to be cleaned up. It's just a premise.
A cave can be built from blankets, pillows, mattresses, table chairs and shelves. Basically, it is a shape that can then be filled by any magical universe: a picnic, a reading cave, a secret cave, a quiet place, a holiday island. Only your imagination limits what your cave should contain.