What you put under a child’s loft bed will depend largely on how old the child is—and how much room your child has to work with in their personal space.
When the child is younger, a play area would work nicely under the loft. You could consider putting a secondary twin bed underneath, for even more versatile bedding.
When the child is older, the area under a loft bed is an excellent space for a desk and a chair for homework and study.